Work out how much you need to cut out remember when the rust stops you need to go about 2 inchs back from it so you get good metal then mark it out then do some cardboard templates of what you need ie right side,left side,bottem,then lay template on your steel a mark out then cut out if you have done a good template you should beable to weld it up in a u shape before going under the car which will save harder welding i presume you have a mig welder if you do not have the right tools mate as said take it to a local engineering firm