It really depends on what you want to run.
In general, learning the individual levels first then stringing them together is a great way to practice multilevel runs like full game. If you want to get better at a certain level, my best advice is to run the level by yourself without any help so that you have a feel for it, then compare your run to someone who has a similar time on the leaderboard. Try your best to copy their strats or at least keep up with what they're doing. You'll eventually get faster, and will aim for a higher standard as a result.
If you're looking at the leaderboards and are concerned with outdated strats, you can sort by the latest major patch as of this post (-- EA (4/15/24) --) in the filters. Note that major patches do not account for hotfixes, so this can still let some outdated strats through (e.g. 5-4), but generally the runs you see are done on the latest version.
As for actual guides, there are a few that I've found really helpful.
For Any%, a lot of people link this video below because most of the routes here are great for people who are just starting out. Just note that since it's an old video, some of the rocket clips are no longer possible on the current version of the game (e.g. 1-2, 2-2). You will just have to do the levels inbounds for that.
A similar guide was also made for the newest layer, shown here:
For P%, there aren't any full game guides out there since it's an extremely long category, but there have been a few layer route overviews. Here they are:
One last thing: you are able to change the version of your game to an older one. This allows you to access faster clips which are mostly used for Any% individual levels. I personally recommend against doing this since it makes the levels less competitive and boring to run, but if you really really want to, there's a guide on how to do that too.
Hopefully this helped give you a direction on where to start. Good luck with your runs! :D
There’s a timing tool in the resources tab. It’s an Excel spreadsheet, which comes with a guide by B.G.B. on how to use it. I’ll link it here for convenience: https://www.speedrun.com/ultrakill/resources/ij9lr
If you can’t use Excel for whatever reason, you can use this instead and it works fine:
I repost this a lot in the ULTRAKILL Discord and the people I've given this video to have always said it helps, so I'll post it here too. Keep in mind that you don't have to record at super high framerates because that increases your file sizes drastically.
EDIT: Adding on to this, you'll want to minimise OBS into your system tray. I find that also helps a lot.