You can complete this exercise in either walk, trot, or canter. But like with most of these exercises, starting at the walk or trot is best. One would usually begin with a 20m circle and spiral into a 10m circle, but you can adjust this as needed. If spiralling to a 10m circle is too tricky for your horse, you can spiral down to 15m.
You want to ensure you do not spiral in or out too quickly but maintain a balanced, calm, and rhythmical pace throughout. This exercise improves your horse’s suppleness, balance, and thoroughness.
It goes without saying that the smaller the circle, the more bend is required and the bigger the circle, the less bend. As you make your circle smaller, you will start to ask for more inside flexion, shift slightly more weight to the inside, and maintain the bend through your horse’s body with your inside leg; you will prevent the horse’s quarters from swinging ‘out’ of the circle with your outside leg. Now, to increase the size of your circle, you will do the reverse; you will reduce the amount of bend and slowly and evenly increase the circle size. You need to keep a feel of the outside of the horse’s body and have a connection between your inside leg and the outside rein. You want to maintain the feeling that the horse’s inside hind leg is stepping through into your outside rein.
The important thing to remember with these exercises is that you are trying to reach a goal with how your horse is using his body; the exercises are the tools that help you achieve this. Ultimately, they are guidelines; you do not have to execute them to the exact metre. It’s all about making progress, not getting something perfect!