These are numbered. Higher number is thicker for less endplay.
I usually start with a 8 or 9.
Now
we can take out forward drum subassembly (left) and direct drum assembly (right) and assemble them together to prepare to drop them in.
Install the thrust washer on the front of the forward drum:
Set the direct drum on the forward and rotate, wobble, spin, until it drops all the way down. It is often easier to flip the drums over and feed the forward down into the directs.
When properly seated it should make a thud sound and look like this