hi,
after you have chosen what you wish your code to be, go to
your shop- then your dashboard - then click on discount codes which is on the left hand column - create new code - (add your code and when you want it to end - it starts straight away- and add the % off your discount code is for).
You will need to promote you discount on social networks such as twitter, facebook etc. and you can also post it on the forum here on folksy. there already exists a few threads for this: