All down to maths (not my strong point) and depends on the wind, but the easiest way I've found of doing it is basically...
Imagine you're flying in a straight line at 100kt per hour. It would take you 6 minutes to travel 10 nautical miles (100kt in an hour, divide an hour by 10 and you get 6 minutes, divide 100 by 10 you get 10, therefore 10 miles in 6 minutes). From that starting point you can work out pretty much anything else you need.
For example, you're travelling at 85kt and there's a 10kt headwind. Your groundspeed (the speed at which you're covering the ground) is 75kt. You're therefore doing 7.5 miles in 6 minutes, so if you know you've been flying for, say, 18 minutes you know you've probably covered about 22.5 miles.
No doubt others will have their own method.
LB.