Go to any train station once you arrive in the U.K. and have your BritRail pass validated. Just bring it to any ticket window and have an official stamp it in the designated area. If there is no official available and you have to leave to catch your train, you can travel on an unvalidated pass for a short period of time.
Write the day and month followed by the year of your first date of travel in ink.
Get on the train and wait for the conductor to come around to your seat or compartment to check your ticket. When asked, present both your BritRail pass and your passport. The names on both must match exactly. The conductor will sign, stamp or punch a hole in your pass to show that you used it.
Continue to use your BritRail Pass in this fashion until midnight of the last day it is valid. Make sure to write the correct date on your pass for each day you use it. This makes the conductor's job easier and can keep your trip running smoothly.