Go to the Booking you wish to create an Activity for. Click on the Activity tab.
Here you will see a list of all Activities associated to this Booking. Click on Add Activity to create a new one.
This will open the standard New Activity window, with the Booking pre-filled in.
Select the type.
Select the priority.
Fill in the Subject and the Content as appropriate
If you want to associate the Activity with a specific Unit, you can search for a unit
Select the appropriate unit
Use the type ahead functionality to select an Assignee from the users in the system
Chose a due date
If you wish, you can also schedule a date and time for the Activity
You can also set a duration and block availability of the unit if you wish. Save the Activity.
Go back to the Booking and click on the Activities tab.
You will see the Activity you just created.