Did some digging on the body work and was ecstatic to find only half my wheel wells

Last person did a bit of a bodge cover up and just filled it all over on the arches. The inside was covered in the horrible rubber sealer stuff so couldn’t actually see how bad it was when I bought it

I’m getting my hands on a mig soon and I’ve ordered the arch and lower quarter of the side panel I just need to make up the inner wheel well with sheet.
Thing is I’ve never welded before lol
Any tips or knowledge at all? Heard some stuff about using a flange tool but have no idea to be honest
Anything is a help