Although we've covered How To Optimise Your Build in great depth, learning the basics of Division 2's build system can be confusing. Fortunately, it's fairly straight forward if explained in a simple way. OMGDracula has done just that, and nails a simple overview.
Choose the weapons that you will want to use for both your primary and secondary. It can be any combination of weapons, but I usually suggest as a secondary you select something that's supported by your Specialization. For example, as a Survivalist, I use an Assault Rifle and Shotgun, as the spec provides buffs to both.
Now that you have your weapons chosen, now is the time to take a look at Brand Sets and see which bonuses they give you. If you take a few minutes to look, you can see which brands benefit your weapons most. You can highly invest into one weapon, or split stats between your main and secondary: that is up to you. For example, Airaldi benefits MMRs and Rifles so you could use three pieces of those, leaving 3 slots open to do variance.
Weapon Mods. The weapons you chose may have bad accuracy or low stability, but these can be offset with weapon mods. If you are lacking on weapon mods, start doing Capture Point 3s at World Tier 5 to unlock some. If the offsets are enough to satisfy you, then you can invest in other armor piece bonuses that don't increase either of those two (such as Elite damage).
Talents are the last step. These can be tailored to your play style just like armor skills. Do you want your talents to be more passive or do you want them to be better in active play? It's up to you. I can't speak for what talents will be meta post balance patch, so just tailor these to your play style.
Attribute balancing. This is probably the only math required which is just simple addition and subtraction, but still not too difficult, as most are simply investing heavily into one stat. If you want to fully invest into offense, defense, or skill power you can go head. However, remember to factor in the Talents you have chose. The one requirement I would say is a high "damage to elite" roll on your mask. This frees up Talent slots to use something other than Hard Hitting.
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