Clean all paint and primer from the areas to be welded. For those with a spot-welding attachment for their welding equipment, skip the next step. If not, drill 1/4" holes in the panel mounting points to correspond with the intended new spot welds. Refit the new panel to the car, double-checking its' alignment with door edges, bonnet , headlamp surrounds and anywhere else it has to fit. Clamp it in place with vice-grips, small g-clamps or whatever fastening method you are using. In awkward corners a small self-tapping screw might be easier, as you must allow for constant checking of bonnet gaps etc., before the panel is finally welded into position.
If using a spot-weld attachment, follow the makers instructions for power and timer settings. Using a standard MIG welder, set at a moderately low setting, start welding each "button weld" from the lower metal surface, building up a puddle of molten metal which flows into the edges of the hole drilled in the new panel. A little bit of practice with this method can produce consistent welds of good quality and appearance. Don't forget to weld under the front panel or down the inside of the door pillar seam if the original panel was welded there. Appearance isn't quite as important for these areas as long as a good strong joint is made.
When all joints have been welded, they can be tidied up if need be using an angle grinder with a stone disc, with final finishing, etch priming etc., being carried out as normal. A little bit of preparation using stopper over the visible spot welds under the bonnet will produce a smooth finish virtually indistinguishable from factory welds.
