I made this build for all the beginners who just got their [Nataruk] and are struggling with Steel Path, this build is easy to build and use. The first things you need are [Hunter Munitions] with [Point Strike], [Vital Sense], and Viral as it will kill any armored enemy in SP. You can find [Hunter Munitions] from the Ghoul Event that comes every few weeks to Cetus. Next, you need to get the [Galvanized Chamber] and [Galvanized Aptitude] mods ASAP since they are so much better than their regular counterparts but you'll need some Vitus Essense from Arbitration to buy them from Arbiters of Hexis. The flexible mods are Thermite Rounds and Vigilante Supplies. Heat is good since it provides -50% Armor reduction against though enemies, it stuns for control and it is generally good versus everything. You could replace it with [High Voltage] for some CC and more AEO, but I would preferably replace it with a Fire Rate mod because it feels slow for me, or Bane Faction mods because they double Slash Bleed damage. However, you don't need the Bane Faction mods before the enemies become ~250. Also, [Vigilante Supplies] can be kept at a low level since [Nataruk]'s ammo is infinite, and it is only here for the slight bonus critical chance. You could use [Hush] instead or even [Terminal Velocity]. Finally, Primary Merciless comes from Acolytes, and it replaces [Serration] in many Steel Path builds but you'll need Steel Essense from Void Relics and to buy a Pimary Arcane Adapter from Tenshin. Primary Merciless is optional, but the damage stacks are much appreciated as they will make a big difference in killing in SP.

If you follow this build, you should have no problems clearing Steel Path as it's all faction-viable. Overframe doesn't calculate Viral's status of up to 325% bonus damage so it's more than 1 Million DPS instead. Also, Viral is the best status in the game so use it in all your weapons unless you have a replacement for it. Enjoy ;)

Ignore the unnecessary Essay below.

Optional: Vigilante mods can be applied in a sentinel gun, and I will still gain their bonus even if I don't equip any in my primary. Merciless is easier to use but harder to maintain compared to Primary Deadhead; it's up to your preferences.

Disclaimer: I show Merciless above because it's easy to use but I use Primary Deadhead instead since the bonus damage lasts for 24 seconds instead of 4 seconds. It's replacing [Serration] so you need some stacks to get [Nataruk] going.

Short Vigilante Buff Guide:

If you have a sentinel gun such as [Sweeper Prime], then you can take advantage of it by equipping all 4 Vigilante mods in its mini shotgun to obtain the Vigilante mod set bonus. It will give you jailbroken 20% critical enchantment, which pairs well with any primary. Jailbroken, as in that you don't need to equip any Vigilante mods into your primary to retain the bonus, and equipping more vigilante mods into your primary won't increase the effect. However, you can equip Vigilante mods into your warframe if you want, Vigilante Pursuit is quite good.
Carrier Prime build: https://overframe.gg/build/468562/ Replace it with Panzer or Djin for SP Endurance.
Sweeper Prime build: https://overframe.gg/build/485975/

Noob's General Modding Guide:

• Use at least 1 Multishot mod in any build because it has the least diminishing returns per mod so you can slap as many as you want and not suffer much.
• Fire Rate is appreciated similarly to Multishot because it has fewer diminishing returns than usual, so try to use one if you have mod space for it. You'll have to take reload speed, magazine, and ammo into consideration. I for example considered that those new to [Nataruk] might not be able to land a golden shot with a Fire Rate mod equipped, etc. Also, many players would rather use something like Arcane Acceleration, Strike on their warframe instead, or a Fire Rate/ Attack Speed Ability like [Wisp] or [Gauss] because more speed looks good. I personally run [Primed Shred] and Critical Delayed together, but it takes a bit more practice to land golden shots with this. Another rule is always using Point Strike over Critical Delay unless you have a Fire Rate mod equipped first to balance the Fire Rate.
• [Galvanized Aptitude], [Condition Overload], and their similar mods are Steel Path Mandatory unless you are using AEO weapons. The bonus damage damage doesn't affect the AEO, so don't ever pair this with Fulmination.
• Slash is the best damage to have if you don't have a 100% Amor Strip Warframe. If your Slash composition compared to both Puncture and Impact combined is 60% or greater then use [Fanged Fusillade], [Maim], etc. Or, if your base Critical Chance is 20% or greater then use Hunter Muntions with at least 2 Critical mods like [Point Strike] and [Vital Sense], 1 for Critical Chance and another one for Critical Damage. Also, Slash is only amplified by base damage like [Serration] or Merciless, Viral Statuses, and Critical Damage Multiplier but it's not affected by Elemental mods including Slash ones. [Fanged Fusillade], [Maim], etc. are mostly used to increase raw damage and increase the proc chance of Slash over a different status such as Viral.
• Always use 60/60 mods for Viral, it's the best status in the game because it provides up to 325% bonus damage at max 10 stacks. Generally speaking, we want to have Viral lower compared to our infinitely scaling DOTs such as Slash, Heat, Toxin, Electric, Gas, etc. As always there are exceptions such as if you are using Hunter's Munitions which doesn't depend on status chance and only relies on Critical Chance to work. Also, I like many people prefer to use the mod rule of 165>60/60>90, which is generally better until you max Viral stacks in about 7 seconds which then becomes somewhat inferior to the mod rule of 60/60>165>90. The only way to free Viral from your weapon is to subsume Nourish or use a weaker weapon with a high-status chance solely for priming Viral. If you have a replacement for Viral, then the next best-combined element is Corrosive, if you have Corrosive already use Radiation or vice versa. Radiation is typically used for Archon hunts, Edilon hunts, and any Sentient fights, etc. Magnetic is nice for a progenitor element for priming since it produces large amounts of bonus damage to shields, which is rarely built due to Toxin outperforming Magnetic all the time. Blast remains the worst element in warframe, unless you have a boss in mind that is particularly weak to it like Lephantis then forget about it.
• The best stand-alone element is Heat because it deals so much more damage compared to others. However, there are exceptions such as using pure Toxin when fighting the Corpus because it bypasses shields entirely and will instantly kill them because their health is tiny. Also, there is Electric for those who use grouping abilities like [Mag]'s Magnetize, [Khora]'s Esnare, Larva, etc. which deals damage in an AEO so if you group them beforehand the damage burst will instantly kill everything in that AEO same rule with Gas. Cold alone is generally bad so DE recently buffed it, but it's still kind of bad unless you are running somehow a 165 Cold Viral with a Primary Weapon that can make use of Primary Frostbite.
• Primary Arcanes replace the base damage mods of many weapons, you don't need multiple base damage mods since it has diminishing returns. You either use Serration or Arcane not both, unless you are on a weapon with low ammo/magazine or long reload that can't afford to waste the initial ammo on stacking damage stacks. The biggest exception to this is Faction mods because it's a final damage multiplier to all damage for the appropriate faction. Faction mods also dip twice on DOTs like Slash, Heat, etc. For example: a 55% Grinner Bane will increase Viral Damage by 55% and Slash damage by 110% instead due to diping twice therefore making any Grinner die nearly three times as fast. If you want to avoid faction damage, then subsume Roar as it provides inbuilt faction damage. Also, Eclipse provides even more damage because it provides more multiplicative damage compared to Roar but it doesn't provide inbuilt faction damage so it won't double dip on DOTs. If you're using [Xata]'s Whisper it will dip 4x instead of 2x provided you have equipped a Faction mod already. There is no effective way around this large amount of faction damage, avoiding it makes it much harder than just switching mods in between.
• Rivens are cool, but not worth it on weapons with low disposition unless they are both good stats and solve a problem like a lack of Viral, low Magazine, or long reload time, etc. They are mostly to even out high-disposition weapons, so I don't invest in them unless it's for a weapon like that. I have some such as for my [Nataruk], Feralx, [Phantasma], and [Tenet Arca Plasmor] since I rolled them by chance. I once sold a [Nikana] Riven for 2000p, despite the low disposition of the weapon making the Riven only marginally better than the actual mod so I would suggest you sell the ones you don't use much. The general rule of thumb is Multishoot, Damage, Critical Chance, Critical Damage, getting 1 makes it okay to use, get at least 2 and it's considered a good roll, but if you are getting 3 it's considered god-like. 4 with a negative to something like Zoom, or Impact if there is no Impact in your weapons is considered god-like too. Some Incarnons that have 2000x damage on non-critical hits want negative critical chance instead, also incarnation rivens sell more than regular so watch out for them. Okay to use Rivens = x-x0p, Good Rivens = x0-x00p, Godlike Rivens =x00-x000p.

I hope I didn't overwhelm you with this info dump. This is a guide I made in hopes of helping those out who don't understand what's good for modding. For level 9999, you have to play with the weapon to understand the weapon's weaknesses and strong points and narrow down the mods into a better build from there. Enjoy :)