1. The easiest way is to highlight the words you want to link and then click the "link"
button in the row of buttons between the "Title" and "Text" boxes. Then, paste the web address that you want to link to in the resulting pop-up box, click OK, and it's done.
If you don't want to use the buttons (or if you're using a version of DU that doesn't have them--I'm not sure if the mobile does, for instance), you can type the code, like this (only using brackets instead of parens): (link:http://www.thesiteyouwanttolinkto.com|the text you want to link).