I hate to sound harsh, but EVERY kid hates being on their tummy. You just have to listen to it. If she does not develop her stomach muscles, she will not crawl and she'll get further behind. Many kids are sitting up assisted at this age. My child was ahead in crawling, sitting up, walking and eventually running at 9 1/2 months. I give it credit to our regiment of tummy time. She's 4 now and has a six-pack stomach. (I did suffered for her, and I wish I could do it for myself too. (Sign...)
Anyway, start out by doing 5 minutes at a time, 3 times a day. Then, go to 10 minutes 3 times a day, etc...pretty soon, you'll notice that she will be holding her head up better and will hold herself up on her arms, etc. She's got to get frusterated enough to learn how to move her legs in coordination with her arms and trunk. You can't teach her to crawl, she's got to figure it out on her own. Be tough...it's for her own good.