Find the damaged mortar and bricks. Using your hammer and chisel, carefully chisel out the broken mortar. Take care not to chip any undamaged bricks.
Clean up any loose mortar you have chiseled with a broom. Bricks can stay put unless they are damaged.
Remove broken bricks. Replace broken bricks with new ones.
Mix the mortar in the 5-gallon bucket according to the directions on the package.
Fill the cake decorator bag with mortar. Use a decorator tip that fits well into the spaces between the bricks. As evenly as you can, squeeze the mortar in between the bricks.
Use your jointer to spread the mortar evenly between the bricks. Make sure your new mortar matches the old mortar by beginning to scrape at the old joints and going toward the new joints.