True extension of any gait is only achievable when the horse has true collection. I'm going to assume that you are actually talking about lengthening the stride, which is what is asked for in first level.
To develop a good lengthening of stride, we first of all need to follow the training scale. We need to start with an established rhythm, a relaxed horse and a good contact. From here we need to start doing tiny transitions within the gait, 10 steps forward trot, 10 steps regular trot, 10 steps forward trot, etc. Make sure your horse is on your aids and warmed up to the idea of going "forward and back". From here, we can increase the amount of steps in the forward and regular trot, and increase the stride lengthening in the forward trot and increase the amount of stride shortening in the regular trot. You don't want to think of this as going faster and slower, you need the rhythm and tempo to stay exactly the same in both trots. If you lose it then scale back the amount of forward and back. If you lose contact, go back and establish it and if you lose your horse's relaxation, that needs to be re-established. This is really gradual work and requires the horse to build muscle and process. THey really need to maintain the first three steps of the training scale (Rhythm, Relaxation, Contact).
As for aids, never pull back on the reins, all of the "forward and back" is controlled by your legs and seat. Your hands should stay entirely passive throught this exercise.