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.