Leyland cypress trees grow to heights of 80 feet or more, and although they widen out a little, they retain somewhat of a Christmas-tree shape if left alone. When height is an issue, and you would prefer for them to stay lower to the ground, you can trim them. But this can cause the main trunk to fork and result in a weaker tree if you aren't careful.
Cut out dead branches with loppers or a tree trimmer depending on the size. Make the cut an inch out from the tree trunk so you don't accidentally cut into the tree.
Cut branches that are close to the ground at the bottom of the tree, using loppers to make the cut. This will increase air flow underneath and up into the tree. Cut these an inch out from the trunk as well.
Cut the top out of the tree when it gets to the desired height. Trim other branches below this cut if you want to maintain a Christmas-tree shape. Trim all branches the same amount, such as 6 inches, so the tree has the same form as before, only smaller. Make cuts with loppers at an angle. You will have to continually cut off new growth to maintain this shape.
Cut one side of a fork off the top if it forms. This will keep the leyland from becoming weak and possibly splitting.