Begin by clicking on the "Tools" dropdown, then "Quizzes." Next click on "Question Library" toward the top of the page:

Click on the blue "New" button, then "Section" if you want to create a folder for your questions.

Next click on "Import" then "Browse Existing Questions"

Search for questions with the search bar at the top left, or browse questions from existing tests or quizzes at the top right. Check the box to the left of a question, then the blue "Import" button to add it to the Question Library.