Hi and welcome to CHR. I have done a wee bit of panelwork on the beloved VW's in my life before so here is how I would tackle this. First as Don said, dress up the edge/flange where the bolts go through so that the guard fits up against the body nicely. I would then using some made fender type of washers bolt the guards to the car. Once aligned and tight carefully tack weld every few inches along the seam with a tig or mig, oh I almost forgot to mention,grind back the too areas to be filled and prime before bolting the guards on. Once finished welding,sand back the tacks,prime and then fill the seam using a little filler.