I have twins that were preemies too. (30 weeks) They are now healthy 3 year olds.
Our physical therapist taught us that "tummy time" can consist of different things.
Try reclining on the couch and put the baby on her tummy, on your tummy. Encourage her to lift her head to look at you. Tuck her hands under her chest to give her that extra lift.
Tummy time should not last more than a few minutes each session. Your baby will get tired.
Oh...bumbo seats, on the floor work well too.
In regards to crawling...trust me, when she is ready, she will crawl. You won't be able to stop it! :)
She needs to learn to sit up first, then she will move to her knees, then rock on her knees, then crawl.
Also get on the floor with her, facing her, on your tummy. Again, fold her arms, elbows down, under her chest/neck. She will use them to lift up.
Good luck...you are doing a great job!