I spot weld all along the join, letting the spot cool for about one second before doing the next one so I get good penetration without having too much heat and warping the panels. Patience is important as is consistency. Once you get the rhythm and the right amount of overlap you'll have a good strong weld that doesn't have warping and is easy to finish. I grind both sides back to the same thickness as the original and usually have an invisible join. Most times it doesn't need more than a little massaging (a few taps lightly all along the weld) to lay flat.

Just looked at the Ferguson link, do it just like that! The key is the fit before welding, time spent there will be rewarded at least two-fold when finishing the panel!