Download UK Festival – What should I submit as an invitation letter for a UK visa from India?

I will travel to the UK on June 19th for the Download Festival in Donnington. I have to start the visa procedure as soon as possible. I have read in an article that I should submit a UK letter of invitation to the visa as well as other documents. I do not think the ticket for the concert will arrive before March, and I do not want to risk waiting before.

How can I apply for a visa for the above from India? What should I submit as in the invitation letter?

Thank you for any advice!