Short sessions. Try doing multiple short sessions a day. Babies often will like tummy time for only a couple of minutes at a time -- this is ok-- just make sure you do it often. As they get older and stronger, it will become more fun. Make sure you set up toys and put your face in their line of sight. Never put your baby to sleep on their tummy.
Lay down with her. Get down on the floor with her. Singing and playing games with her will encourage her to stay there longer.