Your private tour guide lead usually leads you in your tour to profit as the maximum of your trip. He shows you and explains the biggest number of monuments in a limited time (the time of daily visits). He also helps you get the best stuff with the best price in your shopping tour. When you are satisfied with his knowledge, language and efforts, you can show your appreciation by tipping him according to your satisfaction.
N.B It’s better to do it in an elegant way, meaning in an envelope with some thanking words.