It is really easy.
1)Make your Jamaican flag
2)Pull the string at the bottom of the Jamaican flag with you mouth 
Hmm…I’d rather not put my string in my mouth for fear that others might use it. I think this method would prove easier:
Or even this: www.kwos.ca - Ken's World on a String - Yo-yo tricks for beginners made simple and easy (ursaminor)
Or you could do a Rolling Star. I know its an off string trick, but can do it on the string by throwing a Trapeze and putting your thumb in the loop and pulling it open. From there, perform it as you would off string.