Gonna keep an eye on this thread because I have been wondering what people do too. I've been hammering out a routine but I'm basically making it up as I go along and I have no idea how effective it actually is.
If it's the beginning of the day I'll go set off the shel-drone and then re-farm the eggs in Deadly Dance Hall. I have the pattern memorized so it doesn't really test my reaction time anymore but if there's something wrong or uncomfortable with my setup, or if I'm too jittery/tired, that level seems to be the quickest way to make those things apparent to myself so I can adjust accordingly beforehand. If I can get through it + grab all the power eggs without a struggle I'm probably good, and if not I can stop and figure out why not.
Then I'll put on an upbeat playlist and just kind of aim drill/practice in the lobby. I also keep a spreadsheet with notes on what I'm working on and how my matches go, so I'll pull that up and refresh myself on what I'm focusing on. The music helps with the mental aspect, I feel. I don't really drill for a particular length of time, I guess until I just kind of feel ready. If the maps in rotation aren't ones that I'm super comfortable with, or if it's clams at all (I always feel like I need extra warm ups for clams, lol) I'll usually take a few minutes and recon those maps too.
Then usually I will hop into one turf before I go into anarchy. This is more a habit to test my connection stability than anything because every so often my internet starts getting flickery and I need to reboot the modem to fix it. I'd rather find that out in a turf war than ranked, hah.
Most of my drills are just basic dualies-practice, so similar to above, but I really like drills that I can make little games for myself with. Like one of my favorites is to go into turret mode within range of several targets, stay still, and fire continuously, seeing how many I can pop before my ink runs out (I think my record is 11 but I get closer to 12 each time). To make it stricter on myself, if I overshoot any target I have to stop and start over. Usually I'll do this in three rounds-- in the first I can only use motion to turn and hit the targets, but I can hit them in any order. In the second and third, I have to hit the targets going in only one direction, using the R-stick as needed to turn, first going one direction and then the other. I'll sometimes do another little game where I'll start on one end of the lobby and work my way to the other end, trying to pop every poppable target as quickly as possible without missing any shots. (I guess by 'missing" I mean 'fired without having the target in my reticle(s)' not the inevitable RNG missing that happens sometimes regardless). I don't know how effective either of these games are for actually testing aim/mechanics since they're all static/predictable targets but they at least get my wrists loosened up and make me feel good/confident when I can pull them off well and that's part of the mental prep, I think.
All of that said though, my biggest struggle with warming up is that more often than not I spend way too long on it. Probably because there are so many days I never really feel ready and I'm anxiously-subconsciously procrastinating actually playing, even though I always have so much fun once I get started and I'll learn so much more that way(!!!). So sometimes I get so fed up with myself for Being Like This that I'll just throw myself in with barely any warming up just to try to get myself to actually play the game. And...honestly I'm not even sure I do much worse in that context, TBH.