How can I learn to draw UML sequence diagrams. Even though I don't use UML much I find that type of diagrams to be quite expressive and want to learn how to draw them.

I don't plan to use them to visualise a large chunks of code, hence I would like to avoid using tools, and learn how to draw them with just pen and paper. Muscle memory is good.

I guess I would need to learn some basics of notation first, and then just practice it like in "take the piece of code, draw a seq. diagram visualising the code, then generate the diagram using some tool/website, then compare what I'd drawn to what the tool result. Find the differences, correct them, repeat.".

Where do I start? Can you recommend a book or a web site or something else?