I think the easiest way to do it would be to go to the "timing" tab in the editor and on the very right corner of the screen after clicking on that, you will see a box that has buttons like "reset" and "tap here!". After you find the music you want to time the metronome to, of course, hear out the first, most emphasized, and most consistent "accompanying" beat in the background and tap on the "tap here!" box each and every time you hear that same beat throughout the music at the EXACT timing of the song. Do that until you see a message like "BPM sensitivity 0" and then stop. If, however, it tells you that the timing is off, just follow that instruction and click the "Reset" button.
By doing what I'm telling you, you should automatically have the BPM and offset recorded in the editor, that's all. Not that hard, right?
How to find a non-copyrighted video, though? Just go to the description of a Youtube video and see if it says things like "Music in this Video" or if there is a copyright symbol. If not, you're good to go. Conversely, if there is something called a Creative Commons Attribution License in that description, those videos are free for anyone to use and DEFINITELY NOT copyrighted, so that's another way. There are many other ways of finding out as well, but that's beyond what I need in order to do MY mapping anyways. BTW, check the Youtube video again that you linked us to and see if the video fits any of those descriptors, because I doubt the video is protected like that, but I could be wrong. Doesn't hurt to check again! (The info in this paragraph came from a few general videos about identifying copyrighted videos in 2020. Safe to say it's the latest version of Youtube they're talking about in those vids.)
Furthermore, with more detailed help on timing, go to the staff listing on the osu! website and find out the guy with the name of osu!Academy Professor, go to his profile and click on the video in the grid of videos there that talks about timing as well. Would help to subscribe to him too, you'll see why afterwards!
Alright, good luck!