To manually allocate a resource to an activity drag the first work hour to be allocated onto the activity.

Normally the allocation will span from the work hour, being dragged to the end of the activity or not assigned work hours, whichever comes first.

Special situations:

• If you want the assignment done only once, you must press the Shift key while releasing the mouse.
• If you want to reassign an assignment, you must press the Ctrl key while releasing the mouse.

This video shows the process.