To ride efficiently you need to be balanced in the saddle, to be balanced in the saddle you need to make sure both your stirrups are equal length
Take the stirrup in your hands
If your saddle has stirrup keepers like this take them off by unbuckling them
Some saddles have this quick adjust feature or they have a buckle the concept is still the same. Locate your slide or buckle and undo it
These quick change slides may take some work to get off
Slide the slide down
Unhook from the stirrup leather
Count the holes
Count the holes before the clamp on the other stirrup, this saddle is at 8 holes from the bottom of the stirrup leather before the two holes for the quick adjust.
Put your thumb on the hole before the quick adjust
Place the stirrup end into holes 9 and 10 in this case. Make sure the metal prongs are fully through the leather and slide the slide back up.
Reattach the stirrup keeper
There you have it now lets go for a ride!