If you haven't already got a welder get a Mig with gas of around 100 amps (e.g. from Machine Mart). I try to 'joddle' the repair piece if I wish to be neat. The joddler puts a step in the repair piece for a flusher fit. Google for the tool.

I normally weld both joins as this ensures a strong repair. Unless you have some nasty lumps (and you will get some, everybody does ) don't bother grinding off.

Make sure all your metal is clean, and I mean clean. Contaminents will mean it spits back at you like a bastard!

Happy welding