After approximately 10 days (or 25 days for American, Canadian and British citizens) we will provide you with a visa reference number.
The second part of the process is your application for the visa. You should either apply for the visa through your nearest Iranian embassy or through an agency such as CIBT. The visa reference number will form part of your application and is valid for 1 month. After your visa has been stamped into your passport you will have 3 months validity in which to enter Iran.