Hang after class and go through all the throws you know so far with the traditional grips you were taught. Then break each one down to the body position from the entry to where you'd load up your partner than repeat that footowork and body positioning without the grips. Do that a bunch of times modifying how you grip to what you want to get, underhooks you said. Specifically at this point don't modify your footwork and lower body entry and loading, because you want the integrity of the traditional body mechanics to transfer over to your modification. You'll find some throws will work, some won't with underhooks, but as you do this you'll start to learn how to break throws down into their different parts and upper and lower body positions that make them work. You'll also realize there are other stronger quicker grips, overhooks that work better for some throws than your original intent. Once you find a combination you like speed it up and see if it works fast and fluid, then have your partner add resistance and counters carefully and try it in your Randori.
Tinker, tinker, tinker....But always test it against resistance.
