To see what attachments and weapons you’ve found, you need to go into the objectives board. At the top are your weapon and attachment collectibles. You’ll also see where to find the ones you have missing.
You start the game with a number of weapons/attachments already. The achievement is to FIND another load on top of them.
For your attachments, you start with 31. Find another 30 for your total to show 61.
Once that’s done, hold
over your equipped weapon in the load out screen. You’ll go to the gunsmith menu, and can swap out attachments in the bottom right of the screen.