Dig a trench 4 to 7 inches deep and just as wide as your limestone blocks. Extend the trench as far as you plan to lay these blocks.
Insert a wooden stake into the ground at all four corners of the ends of the trench. Run string between the stakes to create two lines of string. These serve as visual markers to help you get the bricks straight. As you tie the string, use a level to check that it is level.
Lay your first limestone block at the edge of the trench, positioning it straight.
Lay your other blocks in line next to the first block, leaving 3/4 inch between each block. With every block you lay, position it straight and level using the string to guide you.
Mix limestone mortar in a wheelbarrow. Use 6 parts brickies (bricklaying) sand to 1 part lime and 1 part cement. For example, combine 3 lbs. brickies sand with 1/2 lb. each of lime and cement. Stir the mortar mix with a trowel. When ready, the mortar will be fluffy and wet but thick enough to hold its shape.
Spread mortar between the blocks, using your trowel to push it in the 3/4-inch crevice between blocks. Let the mortar set up for 20 minutes.
Spread 3/4 inch of mortar on top of the first layer of bricks, while you wait the 20 minutes for the side mortar to set.
Wipe off the excess mortar from the block side joints after 20 minutes is up, using a damp rag.
Lay your second layer of limestone blocks, again leaving 3/4 inch between each block. As with the previous layer, mortar the joints when finished. For this layer, use a half block at one edge, followed by a whole block. Use whole blocks for the rest of the way, and a half block at the end. This achieves the staggered look of the bricks and makes your project stronger.
Continue in this manner until you've mortared all your blocks.