Layering the sandwich. I hate that part; I have back issues and even with my new method working up on a table and using Elmer's for speed, I am sore afterwards.
OOOh, wait, I might need to change my answer to pre-washing. If I had a fairy to pre-wash all my fabric and fold it all neatly and wrap it on my comic book boards for me, that'd be pretty cool. I *really* love having my fabric all ready to go like that, but it's not a fun part of the process for me.