Set up a good regular Working trot, with plenty of energy. Using the long sides of the arena, count the number of steps you make between H and K and then between F and M making sure you can ride the same on both sides.
Now, keeping the same impulsion and regularity, work at taking 1 step more on each side. If you are good at it you can then make 2 steps more.
Use the exercise to improve the canter as well.