When you are logged into your account:
Click on "Documents".
Choose the type of document you are uploading.
You can either drag the photo into the dotted box or you can click/tap the box and find the document you wish to upload.
Be sure that your ID document is valid/in date and your Proof of Address document has been issued within the past 3 months.
Eg: Upload date is 22/06/2019, you cannot upload a document that dates before 22/03/2019.