How would I go about shipping a digital keyboard/piano?

I would go to a music store that sells such things and ask them how they pack and ship them (they may have extra shipping cartons complete with the styrofoam from a recent order). I would then use UPS or Fed Ex to ship it.

Take it to UPS and have them package it for you. It’s a little costly be they have everything and do a good job (in my experiences anyway).

You are probably better off with with what @Kayak8 and @perspicacious suggest, that you have it packed professionally at a shipping store. But… if you are looking to save money, you might be able to pack it up by making a box out of two smaller boxes. Just make sure you have plenty of peanuts and bubble wrap. I have also found that FedEx ground rates are better than UPS rates. And don’t forget to insure it.