Use something stable like a step ladder or bench. Place one hand firmly behind where you are going to sit and with your front hand grab the bridle (if you have one) and place your hand at the base of the horse's mane, and hoist yourself up. Be sure not to pull the horse's mane or he may buck a bit and throw you. Good luck!
grab a hold of the horse's mane with your left hand and pull yourself on top while throwing your right leg over (kind of like clearing the bar in the high jump). It doesn't take many attempts to perfect this move. Once you have it down you will easily mount your horse.
If no bridle then grab onto the main and jump onto the horse this can be hard if you are not physically fit, most people can not lift their legs that high and jump up high enough to put their butts on a horses back, it is hard.