Ghost bookings are suggestions based on your task settings. In other words, when setting start and due dates for your tasks, those dates will be reflected on the scheduler as ghost bookings (those with a ghost icon in front of them).
To turn a ghost booking into a regular one, click on it first. The User, Project, and Task fields will be selected by default and take after their parent task.
What's left is to specify how many hours the user should work on this task under the Hours/Day field → Annotate something if necessary → Convert to Booking.
The ghost booking will become colored, signalling that it's now a real booking.
NOTE: The new update brings the Show Availability dropdown, which allows you to see the available budget vs. the total estimated budget at a:
Booking range level
Project budget level
Task budget level
Before converting the ghost booking into a real one. This acts as a control point so you don't overburden your team.