I'm installing a steel prehhung exterior entry door for my shed build. This is a great preowned/ used door that I found on marketplace for $80 and I'm installing it in my newconstruction setting. This would be the same process for a fiberglass door as well.

Same or similar products I'm using (Amazon Affiliate links):

Nashua Flashing Tape:
https://amzn.to/3MHzUQG

DAP Dynaflex 230 Caulking:
https://amzn.to/46cX9c8

Low Expansion Spray Foam:
https://amzn.to/3SDs5Pt

House wrap:
https://amzn.to/3MHzQAq

*Disclaimer: I'm not a professional, just a seasoned DIYer. You should double check my measurements and advice and adhere to your local building codes.