during the game you will get quests that will give mods one of the first ones is nail mod.
you will need a
or a stick
2 nails(can be found in various containers all over game)
after you have these items and the nail mod from quest go to a workbench and create a item using the nail mod this should pop the achievement
note if you don't have the stuff for nail mod you can get achievement from building any modified weapon.