Hope somebody can help,

I've never done any welding before but I'm going to try it on my car that has just failed its MOT, dont worry its not a ford, its a crappy old merc 190!

Has anybody got any tips?

This may sound stupid for someone thats about to atempt welding their car but when i cut the rusty bit out do I make the new bit sit inside the hole, i.e. fit flush then weld the seem and grind flat, or do you put the new patch over the top of the hole meaning that you have a patch of steel proud of the surface?

And if you were welding up holes in the inner wings would you do it from under the bonnet side or from inside the wheel arch?

Any help greatly appreciated