To install a modification, you need to go to the Loadout tab, select the weapon or item for which it was created, and enter the details about the subject.
There, select the slot to which the created modification fits and install it on the object. From now on, this modification – or several of them at the same time will be active and affect the weapon or kit in a certain way.
Note – you can create multiple modifications for a single slot, but you can only install one modification. So, first of all, create mods for each slot, and then, when you get better items or access to new ones, create now mods.