To alter the left stirrup, first take the reins in your right hand. With your left hand take hold of the spare end of the leather. Then, with your thumb on top of the buckle, steer the tongue of the buckle with the first finger, the other three fingers continuing to hold spare end of the leather. Disengage the tongue and guide it into the required hole. Move the buckle up close to the bar of the saddle by pulling down on the inside of the leather, then replace.

You should never remove your foot from the stirrup. You should get into the habit of changing the length of your stirrups by feel, without looking down.