I agree with the above advice. Take out more than you think, because there is almost always more rust on the back of the bodywork that you can't see. If you take out too little then you'll be trying to weld a new piece onto a thin, half rusted body panel. And rust is a nightmare to try and weld to/near. And even if that works there's still a big rust patch lurking on the inside.
Also take your time and do it properly. I've done a few things quick and dodgy and it comes back to bite you on the arse far sooner than you expect. In my experience it take ages to get a replacement sheet bent correctly and trimmed to fit perfectly, but once welded in you never need to think about it again.
I must admit I always have to put a bit of bog over the top to get things nice and smooth again and here it's a balance between:
a) putting so much on that you're sanding all day to get it back to the right level.
b) putting on too little so there are still some spots that are below the level you want. If this is the case just bite the bullet and bog a little more.