I was told to do it from birth, which I believe is true, theres no reason to wait until 8 weeks, just dont leave them unattended in the very very rare case that they get stuck face down and cant breath.
I was told to do it 6 times a day for a couple of minutes, the easiest way for me is after a nappy change, just on the change table, Jasper is 4 weeks old and has started to learn to roll over and he can crawl on his tummy about 10 cms and I do put some of that to having had LOTS of tummy time from day one.
Theres also nothing wrong with having tummy time on your chest, its getting the job done, she's excersing her arms and neck and shoulders and it also does stop them from getting a flat head.
One thing that works for us, is when I put Jasper on his tummy on the couch and I sit on the floor and so when he lifts his head up, we're able to make eye contact and have a 'chat', he seems to like it.
... also... I find Jasper is more willing to have tummy time if he's been fed, because he's happy, but, it does mean he pretty much always throws up... but thats okay, he doesn't seem to mind, I just have to have a towel handy!