Replacing door bottoms

[Aruba1]

I did a search on the forum on this subject but I have not been able to find detailed information about how to replace the door bottoms. From reading different threads, you suppose to cut the rust and use the aftermarket door bottom to cut pieces off it and patch them in. Mine is pretty well rusted out so I am thinking about cutting the whole bottom and install the new piece. I won't touch the outer skin since they are in good shape. Any help is appreciated.