How do I change a corroded steel drain line to PVC?

To change a corroded steel drain line to PVC, first, turn off the water supply and drain the line. Using a pipe cutter or reciprocating saw, carefully cut out the corroded steel sections, ensuring to leave enough length for the new fittings. Clean the ends of the steel pipe and use a transition fitting to connect it to the PVC pipe, then secure the PVC with solvent cement or mechanical fittings as appropriate. Finally, test the new installation for leaks before restoring the water supply.