http://www.geocities.com/kustzeilen/Genaker/

read that. Your sail maker will need to know the distance between each of the points:

halyard block
sheet block
tack block

Also, ironing out the details is much more time consuming than you think. If it has been done before, in my opinion, it is worth any extra money to just use the same sail maker and copy the setup of the persons who already did it.

I have been through the whole process of putting a custom spin on a boat and would be happy to help you with any specific questions you might have.

Matt