Question: Yesterday we (2 Australians) picked up our Iranian Visas from the Embassy in New Delhi. It was all pretty painless.
We first securing the required 'authorization code' from www.uppersia.com, which took about a week and was a simple process. Then armed with the code we simply turned up to the embassy, applied and had the visa in our passports 24 hrs later. I doubt you could attempt to apply for a visa in Delhi without the aid of the 'authorization code' - "you need a number" - was the first question we faced from the (very affable) man on the desk. We then had a enjoyable chat about the weather patterns of Tasmania! I'm happy to report that with the -seemingly mandatory code - it's not a hassle at all.
Answer: For your Iran visa you should fill in the visa form on Iran visa application, it takes about 3-7 working 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.