Where the original spot welds are drill them out with a spot weld holesaw they are tiny and about £30 from machine mart , pilot them first with a 3 mm drill bit first as it has a centre guide on the holesaw , do all them then measure the new sill which should be larger from the factory so you can trim it down , decide where you are going to cut on the car and go for it but make sure you have enough material on the new sill , get a beefy flat blade screwdriver and tap the whole sill off , mine was pretty easy to get off tbh, clean all the metal as new as possible without taking to much material away as they are very thin especially on the pressed curved sections, clean and paint in the sill that's left on the car ,, trim your new sill and test fit , when your happy it's near perfect either use part of your old sill or make a new metal section that's about 1 inch wide and bend it to match the curves of the original panel then drill , plug weld 1/2 inch on the old panel and 1/2 inch left to weld to the new sill so
1. the sill will fit flushed the original panel
2. You've got something solid to weld against and not just blow holes everywhere
get yourself a 10 mm wild belt sander best tool I own and carefully dress the welds back
then use all the 3 mm holes you've drilled and mount the new sill and mark from behind and drill the new sill with a 6mm hole
most important to clamp as much of the work you can and then plug weld all of the holes up and again dress the back , try and keep the weld as flat as possible as you will have less to grind back.
thats it done simple as lol
Cheers paul