Learning how to do back of the envelope calculations is not simple, it needs to be taught and you need to work on problems [1]

Everyone at university not doing an Imposter degree should have had it taught. It's not university level, but it's above average high school level I'd guess.

I'm not sure how you would learn it online?

[1] A classic problem is have you breathed in part of Hitlers last fart? It's not a hard calculation, but is gives you insights to CNN reporting microplastics being found in penises (Yes the Doomers have moved on past the testicles)