For this achievement you need to visit any Friendly Fire and customise a weapon in each of the 7 slots.
Once in the store press
and open the 'Buy/Upgrade Weapons' menu, then choose a weapon in each of the 7 slots and press
and 'Customize'. The customisations are free so choose which ever you like so long as it changed from Deafult.
The 7 slots you need to customise are:
Note: you are given a weapon for all bar the Explosive and Special slots but you will be given those weapons later on so don't waste your money buying one just for this.
The achievement will pop once you leave the customisation menu