Go into the editor of the trip you're wanting to save to your Library.

While in the Itinerary/Proposal builder, click on the ellipses to the left of Edit in the Day panel.

Once you click on the ellipses, you'll be able to click on Save Itinerary to Library.

A menu will then appear allowing you to change the title of the saved itinerary, as well as the title of each day.

Once you've finished, click on Save Itinerary to Library in the bottom right corner. Your itinerary will be saved and available in your library.

Note: Like other library items, it will be saved as a whole, and in parts. This means the entire itinerary will be saved as one item, as well as each day and Place on each day.