I am an Australian/British citizen travelling on an Australian passport wanting to visit Iran later this year. I have just rang the Iranian consulate they advise me to apply three months prior to departure, not particularly helpful given the need to book holidays/flights etc in advance. They also tell me I will need a reference number from a tour operator which was not part of my plans and is news to me.
Fill in the visa form on Iran visa application, it takes about 3-7working days to get your reference number and two working days to collect your visa at the embassy by direct referral or one working week by posting the docs to the embassy. You can pay the visa fee in cash in Iran.
Check the cheapest Iran tours.