Find the book you'd like in the catalog.
Then make sure UD owns the item in the format you want. (Don't request the movie or audiobook version by mistake!)
Click the title of the book. Then scroll to the bottom of the page and click the Request Item button.
You'll then have to log in to Illiad using your UD username and password.
Then just click Submit Request at the bottom of the screen. If you only need a chapter of the book scanned, tell us which chapter/pages in the Notes field before you click Submit Request.