You will be able to see the option to submit a request underneath the relevant assessment in the right-hand assessment block of each unit’s Moodle area.

If you have previously hidden the right-hand blocks, you can unhide them by clicking the Show Blocks button in top right-hand corner. If you are accessing Moodle via a mobile device, click on the arrow on the right-hand side of the page to bring up assessments block.

Under the Extensions heading you will see two links. The first will tell you whether self-certified extensions are available and, if so, how many days extension are available (you can’t click to add these until within seven days of your deadline). Underneath will be a link to submit a request for a longer evidenced extension.

In both cases a new window will open with a short form for you to complete. 
You can see a video walkthrough of this process here.