Great news, we just submitted our I-751 as well!
Some more notes and insights on the process:
The first thing to upload is a SCANNED PDF version of the I-751 itself (with your signatures!).
Second thing to upload is a copy of the relevant Permanent Resident card(s) (Petitioner, and dependents if necessary).
Third thing(s) to upload are the Evidences of Relationship.
Fourth thing(s) to upload are Criminal History (if applicable).
Fifth section is an open section to upload any additional supporting evidence.
PDF uploads have a 12MB limit.
We don't know if there is a limit of how many PDF documents you are allowed to upload. The more the better we suppose.
The system says you can upload up to five (5) PDFs at a time.
We encountered errors doing it this way sometimes.
We uploaded files individually, and that often resolved the issue.
Some PDFs that I downloaded from financial institutions were considered an invalid format. I was able to re-export them, and then they uploaded successfully.
After that, you accept and acknowledge some things, and then process payment (for the I-751, it is $700)
The process took me 30 minutes from start to finish, but that's because we had all of our PDF documents ready to go.
You are allowed to exit and pick up from where you left off (we did not use this)
The rule is as long as you don't stay inactive for 30 days, then you can pick up where you left off. (we did not use this)