i teach it on the ground first. verbally. “back” - thumb in the chest, back up straight. i set visual markers for myself and the horse - a cone and two poles - teach them to back up thru the poles, which will help keep them straight. one cone is the starting, the other the finish. i keep it to 4 or 5 steps only, the length of the poles.
do this until they associate that area with backing up and start to “offer” the behavior and basically perform the entire task and back up thru the pole chute themselves.
with all of mine, they know “back” - i use it daily on the ground if they are groundtied, if they go in the trailer, if they need to move – so by the time i introduce it undersaddle it is already a known command on the ground.
then i try undersaddle. it’s important to first ask them to back up one step and then go, walk forward immediately with abundant praise. i practice every day for a week or so, and put it into my transitions. for instance, i might do a trot halt transition, think of picking up my hands (not pulling back) and lighten my seat while saying “back”. my legs will slide back slightly so each calf presses the flank with each hoof back. one step back and reward. i do teaching backing up a little differently, when they back up they are yielding, i press forward with my hand - for the greener horses i might rest it on their wither when they perform the step, as i do teach the greenbeans that a pat/hand on the wither is a sign of release/reward. rinse/repeat. it helps if you halt along the rail, or even in between a cone or pole.
once you can get one step that is not resistant, try two. eventually i will ask without the verbal command, but in the early stages verbal is quite important. it will take a long, long time to get backing up polished to perfection, which is why i practice a little every day so that backing up is as route and as commonplace as a W/T transition.
i think for many, there are two issues:
one, “back” is not reinforced on the ground.
two, many riders erroneously have used backing up as a disciplinary/punitive measure. so many, many horses do not associate it pleasantly.