Once you have a list of pages or posts you'd like to add to a project, click the checkbox on the title list to select all items or select them individually.
Then click the three dots on the right hand side and then select "Assign to Project"
Another screen will pop up where you can select which project you'd like to assign your pages or posts to.
Click "Assign to Project".
Now you can see your tasks for each page or post by clicking into the post and seeing the list of "TASKS" on the right hand side.
Selecting "TASKS" will take you back to the Project screen.
Where you can see the list of pages or posts that are part of each project.