I sit down and read through the textbook, taking notes on anything that seems remotely important, doing every exercise and problem set (preferably those with answers). I try to really grok the material and integrate it with existing knowledge before moving on. Much slower pace than any of my college courses, but I'm really starting to pick this up.

When I get bored of this, I play some games :) Or switch to another topic briefly.

As far as testing mastery, my only test is if the material in the book starts to become more intuitive. If I can do the calculations in my head or figure out the relevant formulas and equations without referring to old notes. I've also thought about setting up Anki, but for circuits it seems like a lot of work to figure out how to make usable cards.