The ornament is executed by playing with the index finger and strumming with a downward motion (as is normally done). The difference here is that you will be re-enforcing the sound with your thumb whilst strumming. For example, when you begin any bar, you can introduce the ornament or accent to truely markt the beginning of each bar. You can view this as a stronger method of strumming and it gives another effect on the rhythm which becomes noticable quickly.
Give it a try and refer to the video above for additional visual support.
Re-enforced ornaments for the Venezuelan Cuatro.