This photo illustrates how a stirrup leather should be attached to an english saddle.
First thread your stirrup onto your stirrup leather, then insert the tapered end of the leather under the bar, from bottom to top.
next thread the buckle end of the leather onto the tapered end and buckle. (any hole will work for attaching, you can adjust later)
Finally, grasp the lower strap (the one pictured resting against the saddle) and pull with a sharp snap. This tug will seat the buckle up firmly against the stirrup bar (as shown) and help prevent rubs on your inner thighs from the buckle. (good quality saddles will have recessed stirrup bars like pictured, which help keep you from feeling a lump under your thigh when mounted)
If you share your saddle or frequently adjust stirrups for friends or trainers, try tying a piece of thread around your preferred hole so it’s easy to find when you try to readjust. many times stirrup leathers stretch unevenly so it is important to rotate your leathers back and forth occasionally.