I've flown with quite a few folks over the last couple of weeks and one thing they all have in common is that they have trouble accurately judging their distance from a point on the ground. This isn't a skill that's absolutely essential to flying an aircraft safely but the ability to accurately judge the distance between the plane and a point on the ground can help when providing the aircraft's position to ATC or other traffic.
There's a couple of ways of doing this,
Base your estimate on a known distance. Runways or sections of land are the easiest to use when doing this but anything will work as long as you know how long it it. The idea is quite simple, if your known reference point is your home airport's runway, mine is 6000ft or 1nm, try to imagine how many of those runways will fit between you and the point to which you're trying to measure. It's not going to be perfect but you should be pretty close and practice helps.
Use a point of reference on the plane. High wing airplanes with struts like the Cessna 172, 182, or 206 make this easy but it can be done on low wing aircraft too. Find a object that's identifiable on your map then maneuver the airplane so it's about a mile away from the object and off your left wing. See how far up the strut or along the wing the object is. For example, for me with the wings level in a 172 at 2500 feet an object half way up the strut is about a mile away and an object 3/4 up the strut is about 3 miles away. An object in line with the top of the engine cowling out the front is about a mile away. Keep in mind, this will vary with altitude.
Experiment with this type of stuff when you're out flying, it adds purpose to your flight and makes you a better aviator; challenge yourself to be as accurate as possible without using your GPS.