This is difficult to explain over the internets/via text, but I will give it a shot.
Firstly, I will say you should be able to "see" your distance about three strides out. This gives you enough time to adjust (collect or lengthen as necessary) before the take off. Often times we think that we can adjust right at the fence, and the horse isn't always able to do that for us.
I got pretty lucky in being able to see my distances fairly easily, but it doesn't come easily for everyone.
If jumping without supervision is allowed at your barn, set up a couple small cross rails.
Start by trying to count one stride before the take off. You can do this quietly to yourself. So it would be "One." And then the next 'stride' would be the take off. When you've got that down, count "One. Two." Jump. When you're proficient in that, try doing three strides back "One. Two. Three." Jump. If you don't count correctly, add the stride in there, or recognize when you jump too soon.
In my experience, this is a good way to get people to see. ...Hopefully I explained it in a way that makes sense... x.x