Let me break this down for you real quick. The suppressor youre thinking of is actually for your helmet, not weapons, and its pretty straightforward once you know what to do. First up, you need to make your way to one of the tech guys hanging around places like Rostok or the Chemical Plant, but before you do that make sure youve got permission from faction leaders like Colonel Korshunov if youre trying to do it in the Chemical Plant Base, or theyll just turn you away. When youre at the tech station, hit Q to pull up your upgrade menu, and look for the headgear section - thats where youll be installing this Psi protection upgrade.

Just a tip - sometimes things can get a bit funky if your current helmet already has Psi protection built in, or if its part of an integrated suit setup. If the installation seems stuck, thats probably why. The whole process is really about working with the right people and having the right permissions, kind of reminds me of getting car work done honestly. Oh and while were on the topic, this is different from those weapon silencers you might find later - those work totally differently, so dont get them mixed up!