Part time allocation of  a resource to an activity is done by dragging the first work hour to be allocated to the activity and hold the Alt key when releasing the mouse. 

This activates the following dialog, within which you specify the part of the work hour to be allocated.

Available Show the time span available. Time Time span to be assigned.

Normally the part time 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 part time assignment done only once, you must press the Shift key while releasing the mouse.
• If a work hour is fully assigned and you want to assign part of the work hour to another activity, you can do so by pressing the Ctrl and Alt key while releasing the mouse.
• If you want to reassign a part time assignment, you must press the Ctrl key while releasing the mouse.

This video shows the process.