1. Determine your ground speed.
This is an important step. To make a proper descent you really need to know this. For some aircraft this is pretty easy because the DME sometimes will have a G/S readout(ground speed).
If yours doesn't then do this. Use the DME to mark see the distance traveled in 36 seconds then multiply that by 100.
Example: you traveled 3.4nm in 36 seconds. Multiply by 100 and you have 340kts Ground Speed.
2. When to descend.
You should already know your ground speed so now it is time to determine how much altitude you need to lose.
Example: you are flying at 15000ft and the airport you are landing at is at 1000ft. You need to lose 14000ft of altitude.
Divide the desired altitude to be lost by 1000, and multiply by 3.
Example: 14000/1000=14 then 14 x 3 =42
You will begin your descent at 42nm from the airport.
3. How fast should I descend.
You will use this formula to calculate your rate of descent.
divide your ground speed by 2 then multiply by 10.
Example: your ground speed is 340/2=120 then 120 x 10=1200
This means you will descend at 1200ft/min.