This is one other thing I'm not good at. 
We were talking about this the other day

I suggest this (I'm pretty good at descriptive writing) : first set a simple scene (like your dressing up for a party).
Next describe yourself whilst you're sitting at your dressing table. Start from the top and move downwards.
Even the smallest detail matters - like the way 'your wispy hair was draped across your gown'
Then end it.
That should be more than enough for 500 words.