It is rather simple. Ask some colleagues to help you. You just need to -

1) Send out an invitation for time, place and ect.

2) Buy a cake with $$ donated by co-workers. We normally put $$ together and buy a big item from the registry, like car seat ... We will use the leftover for cake. You can also prepare some punch if you want to.

3) Buy a few decorations, balloons....

4) Maybe plan a small game to play. All your female co-workers should know some games.