You could get the textbooks you used in school, and read them again. This time, noticing when you are confused.

Or use alternative textbooks, if available. That could reduce the feeling of "I have already seen this".

Or take a Coursera or Udacity lesson on the subject. They are usually full of exercises = quick feedback.

It's also good to have a positive attitude about learning outside the school. I mean, it's sad that the school didn't teach you better, but in any case, the science goes forward, the school time is limited, so everyone who really wants to understand their subject needs to study more after they leave school.

I think it's very important to be conscious about your questions. If you don't know something, don't go for the first answer but write the question down and revisit it regularly.

Have something you want to actually do/accomplish with the knowledge, and start doing it as soon as you can. Worked for me with programming (unlike physics where advanced topics largely went in one ear and out the other).