Before this I've only done a few panel repairs. Nothing major like this, that's for sure...
I have a good friend who has been guiding me through the process. Unfortunately he lives in Chicago and I live in Phoenix, so the help has all come over the telephone and by his looking at the pictures of my work.
To get straight lines, tape up to the line then block sand to the tape. You have to first tape on one side of the line, then the other.
I also started with bare metal. I was told to spray the whole car in epoxy primer which is very good for long term rust prevention. Kind of like a factory "e" coat. You can then do your bodywork on top of the epoxy primer. That is, you do all your body filler work on top of the epoxy. If you break through to bare metal when sanding the filler (you will) you can just hit the area with another coat of epoxy.