It's a skill like any other. To acquire it, the most important part is consistency. E.g. Always reserve 30 minutes, 5 to prepare the environment (close window, ensure napping place, put sleep mask, earplugs, etc), 25 for the nap itself. After alarm goes off never try to sleep more. Esp. when you do fall asleep on the last 5 minutes, it is too tempting to say "let's add another 20 minutes" but that won't do any good to get the habit and skill.

The brain must eventually realize it's nap time (preparation actions are the cues for starting), and that only has a very limited amount of time to actually nap.