Here's what to do:

While in Songbook be sure that you have enable list edit checked off ( use down arrow on upper right of the screen)

Choose list edit tab then choose "new list"
Then go to book( 1st tab on the left at the bottom of the screen)
Choose the songs you want to add to your custom list.
Once you have placed songs in the list then touch the write button. A dialog box comes up and rename the list from "newlist" to whatever is appropriate for the custom list you're creating. Each time you modify the custom list i.e. adding a song or changing the order of the custom list be sure to hit the write button or your changes will not be saved.