My older son was born a bit early and spent some time in the NICU. As a result, we had the benefit of being monitored by developmental specialists throughout his first year of life. One thing that they suggested for getting a baby used to tummy time is just simply doing it on your lap. Just sit down and have one knee/leg higher than the other. Place the baby across your lap with her head on the higher end. She'll have the security of being with mom or dad, but will be working on those muscles.
Geraldine is exactly right about the focus of TT at this age not being on attempting to grab/interect with objects, but just on developing muscle control for those muscles which do not get any exercise when sitting or lying on the back all day.