I agree with Hal (and why wouldn't I, I bought his book) that you should make sure that your pitching mechanics are solid. Here is a video from a qualified instructor, Bill Hillhouse. He has a pretty good riseball, wicked rise, actually; it almost took my head off.
You want to use one of the smaller footballs (Not full sized) for this drill.
Simply put, grip the football in the back half, little finger in the first lace.
Do your full windmill and throw the football in a spiral to your catcher.
If thrown correctly it will be a perfect spiral but will kind of be pointing a little upwards. If not correct, it will wobble and flop over all over the place.
To throw a football underhanded to where it spirals, you MUST bring the fingers to the inside of the ball. This is not exactly the same wrist action as a cut rise or a bent fingered rise, but it is very close.
If you are just beginning to work on the riseball, this will get you very close, very quickly, AND it's a nice break from the normal routines and it's a lot of fun too.
An experienced pitcher can sometimes find their riseball going a little flat for whatever reason. It is good to keep the football in your ballbag. Pull it out and throw a few spirals before the game and this can many times fix some minor problems too.
A note to all the Dad's out there;
If you can teach your son to throw a football overhand in a spiral, you can teach your daughter to do the same underhand. Make it a friendly competition and throw spirals back to her, keep count.
People watching at the academy I was at thought I was crazy for having students throwing a football, in a softball school. At least they did until we switched back to the softball and they watched them throw their riseballs. Then, 'Ole Crazy Coach Hal' (and his bag of tricks) wasn't QUITE as crazy as they first thought.