To do a correct pirouette, your horse must take his weight onto his quarters.
On the inside track shorten your horse’s walk while maintaining impulsion.
Without stopping, use your inside rein to ask for a little bend, while holding the outside rein against the neck to encourage his forehand to move around and to control any forward movement.
Your inside leg should stay at the girth to encourage bend, while the outside leg (held slightly behind the girth) asks him to step around.
Once you have completed the pirouette you move forward in the same active walk you started with.
Repeat the quarter pirouette and walk steps so that you end up riding a square.
This exercise encourages your horse to sit on his hind legs and carry more weight behind. With practice, he will become stronger behind, making him more capable of collected work.