The first thing you need to do is queue up for a co-op game playing as the resistance on the "Maz's Castle" map on Takodana. (Not the "MC85 Star Cruiser" map)

Head inside Maz's castle once you spawn and make your way up the stairs to the second floor, then enter the first door in the long hallway to the left. Inside the room you will find a portrait of Hondo Ohnaka, shoot it and pick up the coin that it drops.

After picking up the coin, bring it to Boba Fett's ship, the Slave 1. You can find the ship by going left after exiting the castle's main entrance.
Once you find the ship, look around and you will find three button things that you can interact with, these must be pressed in the correct order to unlock the next step.

Now, I found interacting with these to be a little janky, so be careful not to proceed to the next step before you hear a sound effect play off after finishing the sequence.

The order is as follows:

1. Left
   
2. Right
   
3. Middle

Now you have to run back to the castle and locate the 501st flag, the one with a stormtrooper print.
Carefully position yourself on top of the flag and you will be able to interact with it, do so to move on to the next step.

The final step is to make your way to the basement and interact with the very last door to the left in the hallway.

Woohoo! The skin is now yours and the milestone is out of your way for good, well done :))

Uhm this is like two years late, but thanks a lot for making this guide the top rated Battlefront II guide on steam, I appreciate it <3