Practice, practice, practice. When you are out for a walk, pick an object and guess the range, then pace it off.
In the woods, I sometimes break it down into 5 yard increments. "That tree is 5 yards away, another 5 to the rock, another 5 to the next tree, etc.
I've seen and read of events where a stake is set out in a field and shooters are asked to estimate the range. I can't remember any of the details, but, as an example, it would be something like 125 yards and the guesses would be everywhere from 50 to 275 yards! I know of a couple hunter's safety instructors who have the students write their estimate on slips of paper and then later read the numbers anonymously to humble the class.
My backyard archery "range" is 25 yards and my backhill gun range is 50 yards, so I get a fair guess on objects at those distances.
If you sight in so a roundball from a rifle is 2" high at 50 yards you should be able to aim at the center of a deer's chest and hit vitals anywhere from 0 to 100 yards (point-blank range). With flatter shooting heavy loads maybe a bit farther.