Try it outside
When your horse is first learning, it can take quite a lot of space to get your horse to lengthen his trot.
By doing it outside on a hack or in a field, you can make sure that you give your horse enough time for the penny to drop without having to bring him back to a slower trot for a corner or the end of the arena.
Stop when your horse loses balance
You might get very fast steps, but if you feel your horse starting to rush on the forehand, becoming uneven or trying to break into canter; you’ve lost the balance in the lengthened trot.
If you feel the beginnings of any loss of balance, half it if necessary, ride a small circle before attempting to ask for lengthened steps again.

Aim for bigger, not faster
It can be hard to tell the difference between bigger, lengthened steps and faster steps.
Mentally, it can help you to think of walking up steps; if you walk up a staircase going one step at a time then that’s a working trot.
If you go two steps at a time, that’s your lengthened trot. Your steps are bigger and more ground covering, but you aren’t running up the steps and rushing.
Shorten the stride beforehand
Before you ask for the lengthened steps, it’s important to make sure that you engage your horse’s hindquarters and shift the weight backwards.
If you want to lengthen down the long side or across the diagonal for instance, half halt around the short side and corner before you want to ask for the movement. You need to think about containing the energy and winding it up before allowing it to be released in the extension.