Cut as many of the upper branches as possible, using a pole saw or by climbing up the tree and cutting the branches. A pole saw is a useful tool for cutting through branches high above your head. Depending on the height of the tree, you can climb partway up the trunk and reach several branches with the pole saw.
Tie one end of a rope around a thick section of the tree's trunk and tie the other end securely to a lower part of the trunk or a large branch. Ensure the rope is short enough that the top chunk of the trunk will fall the desired height when you cut it.
Cut through the tree trunk under the top rope with a chain saw or handsaw. When you finish the cut, the top piece of the trunk will fall off the tree but be hung up by the rope.
Move to the rope and either lower the trunk with the rope or allow the trunk to drop safely to the ground.
Repeat this process as you move lower and lower in the tree, cutting off an upper section at a time.