Mix mortar to the consistency of mashed potatoes and apply gently but thoroughly to two faces of a heavy, but brittle object, then place that brick into a corner with both faces with applied mortar of even thickness touching receiving faces at the same time, no lateral sliding. Press the brick gently into the corner and tap, making sure that it lines up perfectly vertically with a string line and horizontally with the other bricks, then wipe off excess mortar, creating a visually appealing groove, all while standing on a muddy slope. Best of luck, robots.