First thing is to put the kettle on 8)
Stand back and have a good look at the job and see where the rot ends and the sound metal begins, then start cutting
When half the floor is missing and you feel like cutting your wrists you are ready for repair panels :D
By now your tea has gone cold and is full of rust and chunks of rubbish :roll: time for another
Make sure you have cut all the rust out and cleaned off all underseal as it will not weld rust and waxoil is flammable :roll:
Remember your gloves, overalls and mask (us welders dont wear them cos we ugly) arc eye hurts, the best way to describe is it is like someone pouring burning ashes in your eyes, then in the morning it feels like superglue has been poured in and you need to peel them open.
Oh now you need to weld it :roll:
Have a search on the net and find some pics that show good and bad welds and give hints on setting power and wire feed.
Basically if you burn holes turn power down, if it sits on top of the metal turn it up.... if the wire burns back to the tip, turn it up. If it spit and spatters turn it down. With settings correct it should sound like frying bacon and will leave the fresh metal with a 'blue' from the heat
If you can, go to a small sheet metal workshop and ask for some scraps to practice, take a couple of quid for their tea fund and they are usually very helpfull
Hope this helps, (my welding is better than my explaining)
Steve