This is how to get an Iranian visa in Istanbul. Again, this is for British citizens, and it may vary for other nationalities, but this should help.
To get to the Iranian Consulate, walk up the hill from Sultanhamet tram stop, take the first major right and walk down the hill – the Consulate is on the left.
The easiest way to apply is online through an agency such as www.iranianvisa.com – indeed, if you go to the consulate and ask to apply for a visa, you will be directed to this website instead. To apply online you will need the relevant information and a scanned copy of your passport. The application takes however long it takes – mine took 3 weeks, but it was the end of Ramadan. If you are lucky enough to be granted a visa, you will be emailed a code, and the Ministry of Foreign Affairs in Iran will fax an approval to the consulate in Istanbul.
You then need to go to the consulate with this form filled in. You will be given another form and told the fee and to get things photocopied. For British passport holders the fee is 95euro, or you can ask to pay 125euro for the same day service.
Fill in the additional form, get photocopies of the additional form, the info page on your passport and the Turkish visa in your passport, pay the fee at the bank across the road from the consulate, and go back to the window in the consulate.
Hand everything across including two passport photos, and wait for an hour if you paid the extra, or return the next day if you didn’t pay the extra. Obviously throughout the process give sensible answers – probably not a good idea to tick the box for “politics” where the form asks for your reasons for visiting for instance…