To equip Skill Mods, you need to reach a certain number of Skill Power. These can be checked in your equipment menu, beneath your Health. Increasing your Skill Power can be done by wearing better equipment.
Open Skill & Equip Mods To Customize
Once you've reached a high enough Skill Power, go to your Inventory and check your Skills. You will be able to access the Skill Mod options for there. If you have equippable Skills Mods, you can place them into the applicable slots on your Skills.
What Is Skill Modding?
Customize Your Skills' Performance
Skill Mods can change the way your Skills perform. Depending on what you equip, you can decrease your Skill's cooldown or increase its bonus damage, etc. Skill Mods are not static, unlike weapon attachments in Division 2.
Gather From Field As Loot
Skill Mods can be gathered from the field as loot. These also come in different rarities, according to what is available in Division 2.
Skill Mods Have Two Slot Options
Skill Mods have two slot options in Division 2 - Gameplay & Cosmetic FX. Gameplay slots affect how your Skill Mods enhance your Skills while Cosmetic FX slots can change the aesthetics of your Skills.