To unlock this achievement you must apply 30 skins to a weapon. To do this, tab over to the Operators section from the main menu, select an operator, loadout, a weapon and select a skin. you will need to do this for 30 weapons (you can take the same skin 30 times). I got free skins for playing the beta and with ubisofy club (uplay). Every operator has 3,4 or 5 weapons (you can see how many weapons an operator have when you buy them). I needed 5500 credit to buy my operators because I bought operators from all CTU's. If you buy more operators from the same CTU you will need more credits.
prizes for every CTU
1ste operator: 500 credits
2nd operator 1000 credits
3th operator: 1500 credits
4th operator: 2000 credits