For someone self-studying mathematics: Do the exercises. Especially the hard ones. Struggle for hours. Don’t worry about solution manuals. It should be painful.
How about look at the solution, understand the parts. Solve the problem. Revisit, revisit, revisit.
This helped me breeze through engineering and also helped me professionally.
Passing math classes requires almost no understanding at all btw, even getting good grades in them doesn't.
I get the best results from struggling with a given problem for no more than 1 hour continuously, and then going away and coming back sometime after my next sleep.
By struggling I mean “not making visible progress” - if progress is happening then just keep rolling.