Before going completely stirrupless, really work on keeping your lower leg rock solid and supporting your body with your lower leg- while still having stirrups.
Then, drop one. I would recommend the outside stirrup because otherwise the horse may react to your outer leg pushing him off the rail. This really helps you balance. Remember to stay tall in the saddle and to use your lower leg. Keep your toes up!
Once you think you're ready, try stirrupless a little bit, even if it is just the sitting trot since you still need to use your lower leg. Then try posting a few strides. And remember to continue posting while finding your stirrups again!!!!
There's a reason why there is a month dedicated to no stirrup work (No-Stirrup November) It helps strengthen your legs lots and keep your leg solid, which is very helpful for showing, jumping, and giving clearer cues to your horse. Let me know how it goes!!!! (This is how my trainer had me start this stuff!)
Intermediate Rider~Learning Equitation and English Pleasure~Interested in Eventing
A horse will tell you everything you need to know about him, if you listen.