Whenever you open any case you will get some skin parts. Once you get enough skin parts you are able to unlock a skin and equip on you Guns/Armor/Mask.
To use unlocked skin follow the steps:
1) Go to Arsenal.
2) Select Guns/Armor/Mask on which you want to apply skin
3) Tap on the unlocked skin and equip the skin.
You can directly buy some unique skins from the store.