If we're talking about a skill that can be written down, the following works for me:

- read the manual and summarise as I go

- using the table of contents of my summary, I rewrite the summary in another document

- I repeat this rewriting the summary a bunch of times over multiple days

- this works for text, diagrams and code

For me, the act of trying to recall from minimal input (a topic heading) gets the job done.

It's really just a variation of the techniques already mentioned here.