Well first make sure you are 100% confident with the smaller jumps. Raise the height slowly, and don't do it it you are really nervous. Remember your horse can feel if your nervous and he will be nervous aswell, and might refuse.
This is an extremely popular question, and the best answer I can give you is to first make sure you're condfident over the smaller ones. Make sure your foundation with over fences is solid and you are great at jumping lower ones.
If your confident with those involve yourself with a trainer who can confidently help you grow and coach you up to higher fences.
If you have any other specifics, feel free to contact me!