I count the strides up to my jump, so I go, “1, 2, 1, 2...” and then when I get to the distance that feels good I squeeze and go over the jump. It has worked for me really well with my horse who really needs a strong squeeze. Counting makes it much clearer in my head, not counting confuses me as to when I need to squeeze.
I usually work on very small fences first, even if you are used to jump higher. You don't have to focus on the height.
You can place some ground pole at the right distances (ask your trainer if you are not sure about the distances for your horse), to visualize when it is the right time to jump.
But for me, the most important part of the work is not just before the jump, if you try to act just a few strides before your fence, you have greater chances to make a mistake. The most important thing is to work between the fences, in the turn before for example, to have the right canter...
So that when you are 3 strides away from the fence, you can look at the next one, and don't stare at the jump in front of you.