Do it as soon as you possibly can, trust me. My LO had a very flat spot on his head because we didn't do tummy time until much farther down the road. It's better to start early and get them used to it!
All of this. As soon as the cord falls off, I'd start it. DD has a flat head so we absolutely make sure to do TT. It's hard because she hates it, but I really push it. If a flat head continues sometimes you have to get a protective helmet which I think would be a major PITA.