During the first year, trim the top and sides to start forming the shape you desire. During the second and third year, trim just the side branches that have wandered out too far to maintain and encourage foliage density. Leyland Cypress pruning changes once the tree reaches the desired height. At that point, annually trim the top 6 to 12 inches (15-31 cm.) below the desired height. When it regrows, it will fill in more thickly. Note: Take heed where you cut. If you cut into bare brown branches, the green leaves will not regenerate.
Read more at Gardening Know How: Pruning Leyland Cypress – Tips On How To Trim A Leyland Cypress Tree https://www.gardeningknowhow.com/ornamental/trees/leyland-cypress/pruning-leyland-cypress.htm