TLDR: After starting the task, click "Add a document" to select from 12 types of proof of ownership documents. You can view these options in the app and upload your chosen document as guided.
Selecting a Proof of Ownership document
After starting the task, you'll first be given the option to '+Add Image/Document'. Please select '+Add Image/Document' and you will then see 'Document Type'. Tap on the arrows to open the drop down list to see the full list of accepted documents you can upload.
There are 12 documents that can be used to confirm ownership. You can see this list in the app (seen below) and here. You will need to upload two documents from the list, you can do these one at a time.
Once you have selected the document you would like to use you can then choose how you would like to upload it. You can read more here about completing this stage.