Choose your topic. The less you know someone the better (worse) it will be. 

Write about what intrigues you about them. Avoid describing physical features like her eyes or hair. Be attentive to peculiar details if you must talk about features,  say you like the creases in his knuckles or her raw chewed fingernails. 

Since you've spent little to no time together, extract every drop of meaning from your previous encounters. Immortalise these memories in words. Their meanings will appear to grow with time. They won't. 

Be cute. Be sincere. If you have no talent then always always be quirky. 

Write on  a typewriter if you can. This makes the worst of poems automatically worth musing over. 

Practice reading your poem in a flat toneless voice. It will mean more to the people listening. 

Never let anyone listen to it and destroy all evidence.