Need UK visitor visa information applying from USA

[ag842017]

I live in the US but hold indian passport.
Status in USA - Permanent Resident (Green Card)
Want to stay in UK for 8 days as visitor / tourist.
Please let me know the procedure / process to apply for UK visa from USA for green card holder.
Need to go to UK around April 2018 - when we should apply for UK visa.

Thank you for your thoughts/advice!