When do you have to start your descend?

Here is a nice tutorial for you:

However, I do it my way.

Let’s say I’m on an FPL and I have 06:32 (6 minutes 32 seconds) before I reach the next waypoint. If I’m at 30,000 feet, and I want to get to 20,000 feet by then, I pick a decent glide angle, say -1,800 V/S. I just multiply 6.32 x 1800, which is 11,376. 30,000 - 11,376 is 18,624 feet, which is the altitude I would be at.

Then I just set my altitude to 20,000 feet to make sure I don’t pass it. :)