There really is no cheap and easy way to do this. If you do a search on this forum and the web you find plenty of threads and pictures of various methods from mild to wild. Keep in mind the gas tank is right there also. The most common of the "correct" type of repairs is to take the tub off and strip the chassis down to the frame. Then blast/sand/grind your way to clean metal. You can then cut back past the rust, cut patches of similar gauge metal and weld them in.
At that point you can decide what to do about preserving the frame. there are plenty of opinions about that as well.
Check out the linked thread post #10 shows my solution. I used scrap 2x3 mild steel cut into a U shape and welded in with a good 110 stick welder, good joint fit up and good preparation. So far so good! You don't need to be a professional welder, but you have got to understand the basics and have some practice.