Just use it a little bit

Think of some calculations that would take a while to do by hand and use LibreOffice and Excel to do them. Look up how to do each part you get stuck at.

I couldn't give advice without mentioning LibreOffice because the cooler boss will hire you for that instead of Excel and they're basically the same thing, pretty sure you can even copy and paste between them, so it's easy to practice with one while practicing with the other.

Bonus points if you make a graph out of whatever data you're messing with at the end.

If you can't think of anything to try, you could look for any infographic out there that's outdated or missing something and try remaking it. A lot of them also have csv data available, like the crypto comparison chart I posted a while ago (I made with libreoffice instead of excel but still csv)

Save the deeper studying with YouTube instructors etc for after you get hired by someone who's gonna have you using excel all the time