TLDR: Each source requiring evidence has its own upload task shown in your task list. After starting the task, you'll see an introduction and supported document types. Upload documents as photos or PDFs, then save. You can add multiple documents by selecting "Add document." When finished, click the blue "Submit documents" button to return to your task list and upload evidence for other sources.
Uploading your evidence
Each source requiring evidence will have its own task to upload it. These will be displayed on your task list.
After selecting to begin the task, you'll see a quick introduction and a list of the supported documents for that task.
You'll then be asked to add a document and choose which type of document you are uploading. You can then upload this as either a photo, or a PDF saved to your phone.
Uploading more evidence
After adding your document and saving this you will then have the option to add more documents for this source. You can add more evidence by selecting 'Add document."
Once you have added all documents for this source please select the blue 'Submit documents' button. You'll then be able to return to your task list and upload evidence of any other sources.