To assign a favorited text, first go to your "Favorites" page. To get there, click on the "My Classes" tab in the navigation bar and scroll down to click on the "Favorites" option.
Click on the text or texts you wish to assign. This will take you directly to the main text page. At the top of the screen, above the text title, you will see the "Assign Text" button. Click on this button and fill out the information to create this assignment.
Unfortunately, you cannot assign multiple texts at the same time, so you will need to repeat this process.