There are many ways to flatten one side for turning.
If you don't have a chuck, do you have a face plate?
- you can turn the wood between centers to round the piece and flatten the bottom (if then going to glue to a faceplate, or make the tenon that will allow you to mount the wood to a chuck., but you don't have a chuck yet).
- I have seen woodworkers flatten the bottom using a belt sander or joiner.
I have turned a great number of bowls mounted to a faceplate. The way I use a faceplate is to mount the faceplate to a round piece of maple or poplar which is a waste block. I turn the waste block round and flatten to top while on the lathe using a sanding block.
The waste block is then glued to your chunk of wood you want to turn.