Hi mate . I’m out somewhere so I can’t write the full letter but I can give you and idea
First u write the format
Address
Date
Name of the person /people
Salutation
1st para ( asking about their health and well being )
2nd para ( all the main content I.e budget,time,plan,places to visit ,things to do etc)
3rd para ( telling them to consider the plan of the trip and contact you as soon as possible with their decision )
Goodbye