You can go through the stirrups but I would hang a D-ring or similar device on the girth strap buckle of the saddle. You can get those type that clip on like a big key ring. Just make sure they are big enough to let the rope slide easily. Make sure your driving reins are long enough to keep you behind the horse a good 5-6 feet. I prefer 7-8 ft.
You put direct pressure on the rein you want her to give to and use the other rein around behind her buttocks down a little. You just swing it a little to tap her with it.
I had never used it until some folks on here (I think it was ALACOWMAN) suggested it for a problem horse I have. I have used it and recently went to a training seminar and saw a professional using it also. It reinforced to me how well it works. It gives the horse training on steering without you having to be in the saddle to do it. You can also work one-rein stops this way. Use one rein and ask her stop, if she does not, tell her to stop, if she does not, make her stop. By this I mean, say "whoa", if no stop, say "whoa" and give a little snatch on the stop rein, if no stop, say "whoa" and pull her head around until she stops. You may have to circle her a time or two depending on the horse.