Adding A Template Over An Exisiting Document

Trying to create a Template...I would like to be able to add form field layers for signing an Invoice where the account information is going to be different each time I use the form...I can create a template in the library but then how do I upload document to place this template over???

I go to Add Template to Library

I press Create a reusable form field layer

I upload the doc and put in the fields and save

NOW, how do I upload an existing document to place this template over???