How to apply for transit visa to visit london

[lydia]

Hi,
This is the first time i am planning on applying for a transit visa.
i am an indian citizen, having a valid indian passport( not expired), currently residing in U.S. On my way back to india..my flight goes through london and i wanted to visit london for 1 day. what is the procedure for applying for transit visa and what is the fees. Please kindly help me.
Thank you