Steps - How to find the Rey skin:
1. Find the Holdo painting in Maz’s castle
2. Shoot it and then interact with it
3. Find the Slave I, shoot the blinking lights
4. Find the three cylinders
5. Interact with them in the order 1, 3, 2
6. Find the Stormtrooper flag on the castle and jump on it, then interact with it.
7. Go to where Rey finds the lightsaber in TFA and once you enter the room you will get it :)