To mortar the center paver in place (optional, but recommended), mix the mortar and add color, if desired, according to package directions. Using a grout bag, apply the mixture between the bricks immediately surrounding the center paver (use a trowel tip to smooth it if necessary). Allow it to dry; wipe away excess with a clean, soft rag.
For extra firming (also optional), mix 1 part mortar with 1 part sand, then brush the mixture into joints between the outer bricks. Carefully sweep excess mortar off the bricks. Mist periodically over the next two hours. Scrub bricks with burlap to eliminate any mortar stains.