An attractive finishing touch to the top of a masonry retaining wall is to install a concrete cap trim, called a "capstone." Capstones are available in standard masonry block sizes, with a variety of edge and surface shapes. The most common edges are square, bevelled and rounded, and standard top surface shapes are flat, rounded or peaked. Regardless of the capstone style you've chosen, the installation procedure is the same for all styles.
Mix an amount of standard mortar mix appropriate to the number of capstones being installed, following the manufacturer's labelled directions. Mix the mortar in a standard mortar box or wheelbarrow to allow mobility and access to the mix during the cap installation.
Access the top of the block wall and spread a long, narrow mound of mortar to the outside edges of the wall on both sides, using a masonry trowel. The mounds should be approximately 1 inch high. Lay out enough to support a single length of the capstone you are installing.
Hold a capstone piece over the mortar mounds you laid out in Step 2 and lower the capstone onto the mortar. Shift the capstone as needed to centre it on the wall to equalise the overhang at each side of the wall. If you are starting at a corner or wall end point, the capstone should overhang at the starting end the same amount as on each side.
Tap the top surface of the capstone gently with a rubber mallet to level the piece. Use a torpedo level to make sure the capstone is straight. Once it's levelled, the mortar joint supporting the piece should be approximately 1/2-inch thick and consistent along both sides of the capstone.
Repeat Step 2 to spread mortar for the next capstone, adding a 3/4-inch layer across the vertical face of the capstone's leading edge. Hold the next capstone over the mortar mounds and lower the capstone onto the mortar. Shift the capstone as needed to centre it on the wall to equalise the overhang at each side of the wall, and press it toward the previous capstone to close the mortar joint between them to approximately 1/2 inch. Tap the top surface of the capstone gently with a rubber mallet to level the piece. Use a torpedo level to make sure the stone is straight. As with the previous capstone, the mortar joint supporting the piece should be approximately 1/2-inch thick and consistent along both sides of the capstone.
Repeat the procedure described in Steps 2 through 5 to install each of the remaining capstones along the wall's length.
At any corners or at the end of the retaining wall, you can use a tile saw or circular saw with a masonry blade to cut the capstones as needed to complete the installation.