Practice. That means both doing problem sets and teaching others (or thinking through the content as if you are teaching, so ask lots of questions and think about how the concepts fit into other ideas).

At the moment I’m working through CLRS intro to algorithms. There’s some math in there and derivations. I tend to run through the derivations separately while I’m working through the content to really internalise them (and I just enjoy the math). The author is nice enough to provide extensive problems to solve too.

Similar can be said of The Art of Programming. Plenty of maths in there to follow along with and plenty of problems to solve.

In addition, don’t be afraid of revisiting content. If you read over a text once but don’t fully internalise it, you hopefully have gained enough knowledge to have an idea of when it might be useful to you. So when you’re working, go back to the text and read through what’s useful and implement that.