How to do warmup?

How do you warm-up? Is there a proper way of doing a warm-up?

Often workouts descriptions only says “do a warm-up of X minutes”, and no details how to do it. Even books by Joe Friel have no details.

If a text actual contain a description, they almost differ in some way. From non-complicated procedures like “15 min with 55% FTP” to very complicated with several/many power steps and different rpm’s.

Even the time for warm-up differ. From 5 minutes up to 20 minutes, or not warm-up at all (for a long endurance workout).

Very confusing.

Problem is we are all individual so no real hard and fast numbers I know of, but I found these videos helped explain it a bit.