Hi,
How trained(well broke) is your horse. Do you have access to a round pen? If you have a well trained mount and it know verble commands you are half way home. Work in the round pen at a walk. Just mount up and then let the horse walk, so you can time up and get the feel of bareback. Then start circles and such all at the walk till you get your balance. You will find the sweet spot of where you sit on the back. The movement at the walk will help put you in it.
Do you know if your horse has ever had a rider on it bareback? Some horses are ticklish when you grip with your legs. Use both upper and lower leg when you grip.
You can lookup video of people riding bareback on u-tube to see the position of their body and how to carry your frame. Just take it slow and don't try to rush it.
Can you keep us posted on how it is going?
BTW, a few years ago I scared one of my geldings. He walked up to the back porch and was bugging me while I was doing some chores. So I slid over on his back, no halter no nothing. Ask him to walk off, and he just froze. He had never been ridden bareback or no bit or halter. Asked him to go and so he walked off like he was walking on eggs. Then walked right onto the corner of the yard where the fence comes together. Buried his head in the corner and would not move. When I slid off of him he let out big exhale. Like he was saying (Whew....) He is better about it now, but it took a while. And this is a been there done that gelding. From being an champion saddleseat show horse, fantastic trail, works cows, teaches kids to ride, lesson horse, horse camp and parades.
Maye
PS. Post some photos of your progress.