It's literally only DIYable by someone who has enough experience that they could do it professionally. Installing your own generator interlock is no easy feat. Installing companion wiring for a generator, again, is no easy feat.

Easiest permanent DIY job would be to wire a dedicated outlet to your fridges and freezers and any important electrical equipment, and switch them to the generator outlet when you need it. No crossing circuits, no real problems.

If you have no electrical experience, deal with the extension cords.