Difficult to tell without seeing the car but for the inner arches think about . .
Cut it out from below - better access also need to cut a bit of the vertical so you can weld to the back of the new patch later.
Cut new patch slightly oversize and bend to fit. Hold in place from below use offcut of timber etc.
Weld from inside the car - from above is above is always easier!
Then weld vertical piece to join good original vertical to your new patch.
Think about access, using an angle grinder from below should make it easier, same for welding from above.
It's scary cutting into your car but I find it helps if you plan it in your head before you start.
Ask if any questions!
Cut it out from below - better access also need to cut a bit of the vertical so you can weld to the back of the new patch later.
Cut new patch slightly oversize and bend to fit. Hold in place from below use offcut of timber etc.
Weld from inside the car - from above is above is always easier!
Then weld vertical piece to join good original vertical to your new patch.
Think about access, using an angle grinder from below should make it easier, same for welding from above.
It's scary cutting into your car but I find it helps if you plan it in your head before you start.
Ask if any questions!