The only thing you need is a sheet of paper. For an ancient and magical look old you might want to decorate it on one side with a spot of ink, fingerprints and red candle wax. Apart from this, you can carefully burn the edges with a fire lighter, making it appear even more like a genuine witch writing.
On the backside of the page you can now write your birthday invitation.
The beginning could be as follows: "Tomorrow Evening at 6 – the little ... witch is coming", or something similar ;-)
Roll up the invitation; put a ribbon around it and that's it – the invitation to the witch party.