1) DON'T DO FLIP P WHIRL! lol Start with drilling the living crap out of flip toe whirl (toe whirl in general is a great trick to learn, it will help with a lot of whirling tricks). What you're doing by drilling flip p whirl is putting icing on a cake that hasn't really been fully made yet. When you get your flip toe whirls down pat, then you can put the icing on it and start doing p whirl.. sugar-free icing, of course
2) So now that you're just drilling toe whirls, the first tip is to SET THE BAG CLOSE TO YOUR BODY and A LITTLE BIT ABOVE KNEE HEIGHT. Experiment with different distance toe sets. You'll quickly find that your leg doesn't like to extend too far or high.
3) When you set the bag, make sure you turn your hips 90 degrees before dexing the bag
4) Dex the bag quickly, pretty much diagonally from your bum cheek and catch in clipper. I find it pretty important to make sure you separate the Whirl dexterity and the Clipper delay, otherwise you'll find yourself hitting these weak ass toe whirls that are really hard to make contact with anything but a clipper kick.
Lemme know if that was clear enough. Hope that helps a little. I actually just drilled 50 flipside toe whirls tonight. Good luck and keep going dude. You're progressing quickly. It's all that basics work