It's pretty easy. Just send a list of names you wish to reserve to the registrar along with $15 per name. Once the name gets approved, the registrar will send what is essentially a registration form back to you except it's marked "paid". When you are ready to name a seedling, fill out the form and send it back to the registrar and youll have a registered iris. You do have to register a seedling within 3 years of reserving a name or you lose it.
John Jones is the current registrar. His contact information and more details about the registration process is here:
http://www.irises.org/About_Ir...
