Short, free, step-by-step guides. Each one covers the manual method first, then the formats the software asks for and the mistakes that cause bad imports.
I also sell a $29 desktop script that automates part of this. It is a Python script, not a website: you need Python installed on your computer and one install command. It does not read scanned or photographed PDFs. See it on Gumroad. The manual methods in these guides are free and may be all you need.