Body/paint floor questions

grind/clean the spot before you weld. mig weld/tig/stick - with sheet metal the process for sheet metal is about the same.
SPOT weld a very short spot.
LET IT COOL before returning to that spot.
You don't return to that spot 99.999% of the time though.
you simply move down a few inches or even better, diametrically opposed welding spots on the piece that you are attaching.
wire brush off your weld every time that you are done to remove any slag.
heat will warp metal so don't get greedy and try to a long run.
practice a few times on scrap to adjust the current/amps and the wire feed on mig. - I always ran too much current on my tig welds though the shape was perfect, I had a rainbow effect on the metal. practice running a line connecting two pieces of sheet metal- practice longer runs than you will really do on your car.
the machine has a duty cycle, it needs to 'rest'/cool-down whatever if you are really moving.
setting up your location to weld into something remotely comfortable will support good welds
if you ever get into tig welding, and touch the tig rod to the metal, CLEAN/GRIND the rod before starting again
Haynes has a good manual for this