The canvas for this is a prefabricated slice of basswood from Michaels. Here is a link. I then sand it lightly and that's it for prep.

I can work with rougher cuts of wood as well, I just need to sand it more. The basswood from the craft store works great and is lightweight though, which makes for much easier shipping. I have a large and heavy oak slice sitting around drying out that I want to make a tabletop out of someday.