This achievement is for putting a mod on a weapon 25 times. You can find weapon mods throughout the game and you can earn them from quests. (one of the first ones you get is called the Nail'd Mod). Head over to a workbench and go to the "create" option. There you can see what you need to make the different mods you can.
You probably can acquire this as you just play through the game, but if not, a good way to collect parts for weapons to mod is by fast traveling to different stores and checking what they sell every so often, because the stock each store has does change.
A fast way to get this achievement is making Deo-Bombs as they only require 2 deodorants and 1 duct tape. Those are cheap to buy at a store plus you don't even need a specific weapon to mod. And well, Deo-Bombs come in handy as they do 5000 points of damage too.
EDIT: Hirsute Dave brought up a good point, Molotovs are also a good thing to make, as they take 2x alcohol and 1x rag. So it just depends on what you have available to you.