Apply 4 different Mods to a single weapon or Warframe.

To apply mods, you'll first need to find mods and level up your weapon/Warframe. You'll find mods all over the place in the game. They are the silver circles you'll see laying on the ground. When you pick one up, the mod will show in the form of a card on the right of your screen. To level up your weapon/Warframe, simply keep using it in battle.

If you open your Arsenal while in the main menu, highlight your weapon/Warframe and press to upgrade it. You'll see 8 squares on the right, and the current mods you have that can be applied to the weapon/Warframe on the left. Click on one of the squares with and pick any of the mods afterwards to apply it. Apply 4 mods in total and press to save your current build.

If you haven't leveled up your weapon/Warframe enough, you might not be able to apply 4 mods. Your weapon/Warframe starts out with 0 mod points. Every time you level up your weapon/Warframe, you'll gain 1 mod point. Each mod costs a certain number of mod points, indicated by the number in the top right of the mod. This number varies per mod. The most common mods will cost you 2 mod points, so in order to apply 4 mods you'll need to level up your weapon/Warframe at least 8 times.

You might also see small icons in the top right of your mod slots. These are called polarities. Mods also have these polarities. If you add a mod to a slot with the same polarity, it will half the cost of the mods points needed. If you add a mod to a mod slot with a different polarity, it will increase the cost of mod points. Adding a mod with any polarity to a mod slot without a polarity will cost you the normal mod points.