I've seen this suggested many times but I've yet to see anyone whip naturally this way. When I think about throwing underhand myself, I certainly dont whip. I push.
First, get the IR correct. The thread "IR in the Classroom" is great for developing the IR motion. Most of the drills can be done as part of pitch and catch.
I learned to throw IR "for the heck of it" when my DD was learning. My DD and I now mostly play pitching and catch IR underhanded. I've been throwing batting practice to kids for 20+ years. (Nothing like a 10YOA girl running up to you and begging you to throw to them.)
As to learning, I'm assuming you know how to throw a baseball overhand. If so, you already know how to IR.
Do the IR in the Classroom Drills with your DD. Basically, get open, take the ball to 12, have your hand behind the ball, stay back, and throw.