I'll keep it super simple.
I'm a fan of Harrow and this is quite simply his best general-use build I use.

General
Harrow's best strength is turning shitty weapons into God-tier ones by amping their fire rate, reload speed and critical chance.
In order to achieve that you have to be constantly spamming abilities and fire headshots.
If being precise and meticolous isn't your forte you might want to look into other warframes

Efficiency
So first off, [Harrow] doesn't need energy as he can replenish plenty for himself.
We can even very safely reduce his efficiency to rock bottom as one of his passives is that he starts with 100% of energy filled.
So two extremly important mods that should never miss in a [Harrow] build are: [Primed Flow] (because it let's you start with a ton of energy) and [Blind Rage] (as it's the most powerful strength mod).

Duration
As for duration, it is useful for Penance because you have to cast it less often and get yourself in danger less frequently, for Thurible as you don't have to cast it as often so you won't get bored, but most importantly for Covenant/[Lasting Covenant].
So... a lot of people don't like this augment because it requires you headshot, but I think it plays nice with the rest of his abilities, if you don't like headshots you probably should look into other warframes anyway.
[Lasting Covenant] basically lets you keep Covenant's bonus if you simply get a headshot kill every couple of seconds, based on your duration.
Narrow Minded is a big no-no as we need range, so [Primed Continuity] and Molt efficiency are more than enough.
I'd advise against swapping [Augur Secrets] for [Augur Message] and using a strength based molt arcane, as neither of the two synergize well with [Harrow], Molt Augmented takes too many kills and Molt [Vigor] is only on the next ability, [Harrow] casts a lot and takes more time to kill, favouring headshots, so less kills overall.

[Natural Talent]
Talking about Covenant, [Harrow]'s abilities are pretty slow, especially this one. So Natural Talent is another fan favourite. It lets you cast thurible much faster too (no additional energy/sec though).
Not to mention that [Harrow]'s biggest threat is himself, as Penance leaves you extremely vulnerable after being stripped of all your shields, so being able to cast Condemn as quickly as possible right after is a Godsend.

Range
The only reason to not have rock bottom range as [Harrow] is Condemn and unfortunately it's a pretty big one at that.
Many people switch it for Pillage but in my opinion it's a mistake, as Condemn gives you free headshots and as already said they play a big role in this build.
[Stretch] is all you need though, avoid [Narrow Minded] and steer away from a needless [Overextended].
Just be mindful of your surroundings, chain every enemy you see and stay away from the centre of attention, if you get shield gated immediately spam Condemn, there's a high chance he'll be behind you or near you and you'll have chained him without even knowing.

Strength
All of the remaining mods should rack up as much strength as possible.
More shields when Condemning are nice, getting more energy/sec with Thurible is too, you won't get further crit chance with Covenant and you'll max it out extremely quickly (especially in Steel Path). So really the only reason to amp strength is Penance.
Having high strength gives you an absurd amount of reload and fire rate and melee speed, letting you play pretty much any mediocre weapon extremely easily.
As you won't need any energy orb and as you'll be casting a lot of abilities, [Energy Conversion] is a clear no, so [Umbral Intensify] and [Augur Secrets] it be.
[Power Drift] is another easy choice, more strength and the only other two exilus worth looking into are: fans' favourite [Primed Sure Footed] (which I suggest not using and simply avoiding radial attack weapons as they won't be good for headshots anyway) and Speed Drift, which I didn't choose because between speeding up your weapon's reaload/fire rate or your abilities, the former wins every time. [Harrow] is a slow and repetitive caster, but absolutely not as much as realoading your weapons.

Variants
You could swap out [Augur Secrets] with [Transient Fortitude] and [Stretch] for [Augur Reach].
The reason being that I've run [Primed Continuity] for so long that I feel like the new arcane Molt Efficiency is a bit redundant on [Harrow] (though believe me there will be times where there are no enemies and you need headshots to keep Covenant running and you'll wish you had a greater slack, especially defenses) so you could opt to get a bit more strength in exchange for a bit less duration.
The problem now though, is that [Augur Secrets] let you cast Penance (which gives you a bit of shields from spending energy and then strips away the amount of shields you HAD before casting the ability meaning you never shield gate unless attacked) without losing Molt Efficiency's bonus.
So instead, I feel like you should swap [Stretch] for its lesser Augur counterpart, which is still tolerable.
You can also, if your build revolves around one single weapon, swap Molt Efficiency for a primary/secondary/melee damage-dealing arcane, you don't really need the fire rate ones as they're very redundant on [Harrow], but I'd still not advise this as more duration is always welcome for Covenant and [Harrow] is one of the few to really profit from that arcane.
I don't like [Rolling Guard] either, you can use it as a panic button, but really all you're doing is delay the inevitable because [Harrow]'s only real escape is getting more shields and chaining enemies so doing anything else other than that feels like a waste of time and you'll gonna get one shot anyway if you don't Condemn, plus with an Augur mod you can somewhat spam Condemn without even hitting an enemy and still be safe for some time via shield gating, you can even shield gate yourself from statuses like electric traps and hyekkas' [Ignis]. Toxin is a bit tougher but you'll probably get one shotted anyway if you get hit by a toxin dealing mob so [Rolling Guard] in my opinion feels like a wasted slot on [Harrow].
Another thing you might want to get is a Decaying Key, as [Harrow] gets double overshields he doesn't need additional normal shields, and with a Decaying Key and this much strength he can chain even just one enemy and get enough shields to get into the overshields and gain back his 1,3 seconds of shield gating instead of the 0,3 when you didn't replenish them completely, thus giving you more time to breathe at the expense of a couple hundred max normal shields.

Critical Chance
This is a bit of an advice but, since [Harrow] is capable of increasing your weapons's critical chance, you'll want ones with the highest critical damage and so therefore additional critical chance, coming from headshotting (which quadruples the max additional 50%) and Arcane Avenger are absolutely welcomed.
Since [Harrow] replenishes his health as well, by killing with Penance active, we can safely use an unranked [Combat Discipline] as aura mod, which makes us lose a bit of health which we gain back immediately, with each loss giving us the chance to trigger much easily Arcane Avenger.
Other popular aura mods include [Brief Respite] (which I strongly recommend against, as it doesn't replenish overshields, meaning you won't get additional shields pretty much all the time and it won't [Protect] from losing Molt Efficiency bonus when using Penance, unless you're on your normal shields, in other words never and you should cast Penance then anyway), [Corrosive Projection] (useful, but really only against Grineer and Deimos, I already one shot anything in Steel Path with [Combat Discipline] and a sniper, so having [Corrosive Projection] work SOMEtimes doesn't really appeal to me, but you do you), [Growing Power] (not a big status fan and the very small 6 seconds delay doesn't make this mod appeal to me, furthermore between more fire rate / reload and more critical chance I choose the second one), Rifle/Pistol/Shotgun/Melee Amps (if you only use one weapon you could actually consider swapping it)

How to play
The first thing you want to do is cast Covenant, as you're getting hit (at around 800-1k damage you can safely assume you've already maxxed it) cast Thurible and get 10-20 energy/sec depending how crowded is the mission (30 is already in the too much territory), at this point you've probably already maxxed Covenant but you're still in the invincible phase, so chain some enemies to get max overshields (you don't need too many and you don't want to chain them all rightaway), you can then [Sacrifice] them with Penance and chain the remaining ones to be safe (if you're skilled enough you can get two Penance in order to max its duration and still chain some more for your own).
At this point Covenant invincibility is over so you can start meticolously blowing heads up. If other players get in your way and start killing your enemies before you're done headshotting them or racking some damage for Covenant or getting some shields/energy, then move away from the objective or simply from them, many people use radial attacks which don't leave room for patience and probably if they're sweeping every room it's probably not needed anyway and you should have chosen a different build.
Thurible and Penance need to be constantly checked out as they run out eventually and you don't want to end up with no energy (always bring some Large Squad Energy Restore for that case) or with a slow realoding weapon (in which case don't wait and immediately cast Penance followed by Condemn, then reload).
Condemn enemies at every chance even if you have max shields, you need those headshots and you need to always need to have max shields for an emergency Penance or to just stay alive.
The only real threat are hidden enemies which you can't see or can't [Reach] with Condemn in time and stun them, so always move and if you're getting constantly shield gated for apparently no reason and can't figure out where the damage is coming from while casting Condemn in every direction (it's best advised to spam itas panic button, you'll have plenty of energy to spam it, you'll gain shields just from doing so due to Augur Secrets/[Reach] and there's always the offchance that you'll hit someone and gain back some shields for a much needed proper shield gating.
Last thing, don't be lazy and be constantly on the move, looking for enemies, chaining them, headshotting them, etc.... this warframe is played best as a sort of minigame where you need to keep up with the game's rythm.

Best of luck.