UK Visit Visa Financial support letter

My younger brother wants to apply Visit Visa for UK and wants to attend Cricket event in near future for 3 weeks he is BS student. I am going to bear his all expenditures. Can someone please guide me whom or how to write a letter(Financial support) for this? I have already booked Hotel and flight ticket.
It would really great if someone helps me with any sample.

Also please tell me what documents are required?

thanks

It is advised by UKVI that applicants NOT book flight tickets until they have their visas approved.

What evidence is he going to send with his application to prove his personal, financial and social ties and circumstances in home country?

How old is he?