Midmotorsteve

Got a old mk1 mr2 as a project, it needs new rear arches rust problem, the drivers club sells replacement arches. How do i go about welding them on? never done an welding before. Do i cut away the rotten areas then spot weld the new panels on overlap, new bits overlapping on outside or inside? fill gaps smooth etc..
i have no idea, generally capable mechanically so think i'll do ok with a bit of guidance

cheers

steve