Best way is to send it in the box it came in
If you don't have that anymore, then it's best to find something similar.
Best if it's rigid in some way. And pack it well.
It's upto you to get it there in good condition.
Take good quality date stamped photos of the phone and with the screen on, to show it's working
I also take pictures of how I've packed it.
Also my preference is to pay extra to have the package signed for on receipt and to get some insurance with the postage