In short, everything.
Practice regimes are entirely player dependent. For me at least, I always try to make note of what tech I messed up at the last local I attended and practice that, even if it's something like missing a wavedash. Honestly, I think some of the most important tech in melee is the basic stuff. Ledgedashes, wavedashes, SHFFL, and wavelands are all super important to be able to do 100% of the time because of how often you do all of them.
In terms of Doc specific stuff, Up-B cancel out of shield, SHBAWD, chaingrabs, and wavelanding out of pills are all really nice tools once you implement them correctly. Definitely practice chaingrabs on every character that can be chaingrabbed. That free percent is really nice.
For stages, it really depends on the character and who's playing them. I find Stadium to be a pretty solid stage since there's no top platform that people can camp. FD is a great pick versus spacies and Falcon cause of how disgusting your punishes can be off of the chaingrab. Every time you grab Fox or Falco on FD you should get them to 70% or kill them. It's straight up like fighting Marth. Thinking on it, I don't know if Doc has any really bad stages. I'd say he just doesn't want to go to stages that offer advantages to other characters. He's not like Puff where Pokemon plays directly against her.
