Getting paid for something you've just learned, is a great way to master it.
Just like teaching it to someone else is.
Because it forces you to think about a whole lot of things you would otherwise not even have paid attention to.
It forces you to deliver professional work.
To be able to answer your student's questions.
Or your new client's questions.
So if you're serious about learning something new, try teaching it to someone who really wants to learn it.
Or if you don't like teaching, then try to find someone who's willing to pay you for your new skill.
It won't be much and you'd have to start at the bottom, as you're not a professional yet, but you'd essentially get paid for getting better at it...