(assuming you're using a mig) Go very slowly, and just do a few tacks spaced well apart, leave it to cool, then a few more tacks - if you run a bead the panel will distort badly due to the heat, and if you do too many spots in one go it will start to distort due to heat as well.  Takes ages, but if you want it as straight as you can get, you don't have much choice.

The way I do it is to cut ALL the rusty bit out, then form up a piece to replace the section I've cut (if its something like a sill or curved or anything), and make it so its a nice close fit all round, and then weld it in flush - the idea being to use the absolute minimum of filler possible, over the ground back welds.  As far as I'm concerned, where possible filler should only be a surfacer to fill small imperfections, and not a way to build up a sunken bit of panel.  If it is a largish section you are replacing, ideally you would have (once welded, filled and sanded back level) filler around the edge of the patch, and then bare metal showing in the middle of the patch.  I weld the whole way around the patch (spot, spot, spot, snore, spot, spot) so that there are no holes/gaps between patch and original metal, then grind it back smooth with a fine flap disc on a grinder (be careful about getting heat into the panel!!!).  Redo the holes that appear, grind, repeat.

That said though, looking at some of the dents in your car, it will be a struggle to get the panel back into original shape anyway due to the metal stretching, and in some spots the access will be very limited.... This may be a good spot to do your first repairs, as the panel will likely need more than the light skim of filler desirable, unless you use some creative panel beating and much patience.