Mk1 Sill replacement

[ClioGav]

My car recently failed the MOT due to loads of rust, L reg mk1 :dapprove:

Anyway here's what i need to replace to get it through!

Sill L/H side : £22 (68-30-007 Beevers)

Sill R/H side : £22 (68-30-006 Beevers)

Rear arch L/H : £16 (816745000 EuroCarParts)

My mate can weld so he's gonna do all the work, I was just wondering how the sills are replaced?
Do the new ones just cover the original sills or do the old ones have to be completely cut out?

Heres how bad the arch is, i know it can be taped over for the MOT but i want it mint again

 

[Maxx]

Looks like you'll prob need new arches mate

 

[ClioGav]

Rear arch L/H : £16 (816745000 EuroCarParts)

I know mate :clown:

 

[Final Maxim]

cut out the bad sections then weld in the new sections. if its fucked all the way along then probly best put in an entire new sils

 

[ClioGav]

Thanks mate