Once the big sections of metal are gone, you must trim away all the remaining edges in preparation for fitting the new sheetmetal. From the factory, nearly all mass-produced cars are spot-welded together. Cut through the spot welds, and the car comes apart. There are a number of ways to do this, and the easiest is to drill the spot welds out using a bit that is slightly bigger in diameter than the spot weld. Oftentimes you will only need to drill through the thickness of one of the two (or more) stampings joined together, so be careful to only drill deep enough to remove the part you want. See how the tab from the front framerail is separating from the Mustang's floorpan as the welds are drilled out? There's no sense in drilling all the way through the floor, creating extra holes you'll need to fill later.