how far along are you? i would get walk, trot, lope and whoa all down pat before starting the back. When your horse has all of these down, start from a stand still. apply light but consistent pressure evenly with both hands and either gently squeexe or wave your legs on the horses sides depending on your prefreance. usually the horse will walk a step forward and toss its head out of confusion. Keep the steady pressure until it finds the correct answer by shifting its weight back. RELEASE IMMEDIATLY. to begin with it dosnt matter if they dont actually step backwards just so long as the SHIFT backwards. Then build from there using the same technique. Remember: it is ALWAYS better to release pressure too early than to hold pressure for too long.
Depends on your prefered training technique. I train all my horses ground driving/long lining before mounting.
@ the very beginning stages your horse should lead well-if you stop it stops if you move left/right=it does, if you face it and say back it should, which can be enhanced to you just stopping and stepping backwards, without turning.
Assuming your past just lunging w/ tack, attach 2 lines to either a halter/sidepull or if a bit is used I would use rubber.
One line goes down either side of horse-starting same as lunging w/ you in center [progresses to you directly behind,blind spot-which is where you'll be in horses vision when mounted] outside line will be longer than one on inside of circle [can be loose just resting above hocks, through surcingle or stirrups depends on horse]
Ask for whoa, say it & just let horse walk into non-giving pressure. Than say the word back and from a standstill apply pressure just from your elbows.
As posted above take a little @ a time you may get full back up steps or just a lowered head, but anything besides resistance should be rewarded by relaxing the pressure.
Do not immediately send forward, just no more pressure is reward enough.
If ground work is already established, it should not take more than 1 or 2 training sessions.