If I’m flying at FL200 and I want to get down to the airport at the end of my descent. The airport runway will be 0 in altitude so how do I calculate my descent.

I found using time is the easiest way to figure out. Using your example I would start descent 20 minutes from the airport at a vertical speed of - 1000, or 10 minutes from the airport at a VS of - 2000.
Since ground speed decreases as you descend, you will always come in early, so you will need to level out a bit now and then between waypoints.

I use the 3 X altitude = distance method too verify I am at the correct altitude