Step 1: Get £24,000 ($31,000)
Step 2: Go to a bank that will give you foreign currency
Step 3: Exchange it for 1,000,000,000 Iranian Rials (rate from xe.com).
Congratulations - you are now a billionaire.
Get lots of photos taken with your money. Put it into a bank and get a bank statement to prove you are a billionaire.
Then change your billion back to real money and carry on your life as before, but knowing, for one brief shining moment, you were a billionaire!