I completely agree with what folks have said in regards to the cross rails. If you feel that she's done enough pole-work, pop over a crossrail here and there (say under 18 inches). Start by trotting over it, just going back and forth from different directions. You may use some trot poles before and/or after the cross rail if you feel she's having trouble with keeping a steady rhythm, but use your judgement. Some horses become more discombobulated by the poles in which case I would not use them. The point is, you want her to have a positive experience and keep things as simple as possible. Once she's trotting over the crossrail and cantering away in a nice steady pace, start cantering to and away from the jump.
Some folks mentioned grid work and to me, this implies gymnastics (i.e. 3 or more jumps in a row) and I absolutely would not do this with a horse JUST learning to jump---it is simply to much for them to process when they're just learning and can result in the experience becoming overwhelming or perhaps causing them to refuse jumps. Now gridwork is WONDERFUL once the horse is confidently jumping lines, as it encourages the horse to take it's time over the fences and jump correctly. However, to start, just keep it simpl and do single jumps (cross rails and perhaps work up to 2'o" verticals) and also incorporate some basic lines (i.e. 2 jumps in a row), where the jumps are at least 5 strides or more away from each other. If you don't know how to space jumps correctly, ask how, as poorly spaced jumps can result in you meeting the jumps at awkward distances and also not allow you and your horse to gain a feel for a proper pace. The rule is to allow 12 ft total for take-off and landing and then 12ft per stride, so a 5 stride line would be 72 ft. If you don't have a measuring tape, the general rule is 1 of your steps is 4ft...usually this is a little bigger step than normal unless you're really tall.
Now 2 other things to keep in mind when starting your horse over fences: There are 2 "sins" a rider can committ when jumping and these are particularly important when your horse is just learning: 1) Never catch your horse in the mouth, i.e. always release your reins over the jump, grab mane if you need to. 2) Never come down on your horse's back over a fence. This requires good leg strength, so work without stirrups on the flat if you have an issue with this and grab mane in the meantime over EVERY fence. Releasing and staying off your horse's back will allow your horse to completely use herself to jump the fence and will encourage good jumping form and confidence which are both so important!
Lastly, one person mentioned to make sure you always come to the jump at the right spot and I'd say the exact opposite. Don't worry about your spots or "distances" at this point. 1) It takes a lot of jumping on your part to develop an eye for distances and you simply may not be able to see or correct distances at this point 2) As a green horse is learning to jump, it's fine if they "miss," they need to learn these lessons so that they learn to find the distances themselves or at a minimum, learn that they can still get over the jump even if they are not at the perfect distance---in other words, stay out of your horse's way and let them learn. What is very important however, if you do miss is to make a concerted effort to not drop back in the saddle or jerk your horse in the mouth when these "misses" occur as that can lead to refusals or lack of confidence on your horse's part.
