Start your horse out by just walking over to the jump and letting your horse sniff it. Get them used to it walk,Trot,canter near the jump. Then when you think your horse is ready try jumping it at a Trot (if it’s small enough) never rush things and let the horse take their time
Make sure they aren’t scared about any aspect of the jump, make sure it’s low and not too high, and make sure nothing about your posture or anything is causing them to refuse. I would also check to make sure your tack fits just in case it may be uncomfortable, and also make sure that when they do go over the jump, don’t pull on the bit or make it to where they don’t want to go over again. Give them tons of pats and praise! Also, don’t anticipate them refusing. You obviously don’t want to fall off, but if you anticipate it, like anything else on a horse, they’ll do it. My horse refused for quite a bit and so I’m pretty familiar with this.
A great rider I have always looked up to said, "you win your rosettes at home. You just collect them at competitions."
It's a long process but it all boils down to building your horses confidence. Start small and work from there. It's not something you do in a day or week. Months, even years go into work like this.
Devin is right though, just be patient and work with it. Good luck!
Don’t look at the ground or at the jump and use a lot of leg and even crop when your about to take off. Look strait ahead and don’t worry about it. All you have to do it go into your 2 point and the horse will do the rest if you apply leg