Take the stirrup iron and slide it onto the stirrup leather.
Then buckle the stirrup leather, by sliding the end through the top slot of the buckle.
Then slide the stirrup leather onto the stirrup bar just behind the buckle on the leather. The stirrup bar is a metal piece located under the small flap on the side of your saddle. You may need a hoof pick to help you lift the stirrup bar and slide the leather on.
Check to make sure that the stirrup is hanging correctly and that there are no twists in the leathers. If there are twists; take the leathers off, straighten and reattach.
Close the stirrup bar. There is a hinge of the back of the stirrup bend it upward. This ensures that your stirrups will not come off while riding. (This hinge may not be present in all English saddles.)
Mount up and RIDE!