Select the Slate
To determine how much stone you'll need, first lay out the path, using stakes and string to mark its boundaries. Then multiply the path's length by its width to figure out the square footage that you'll need to cover with stone. Add 15 percent to the total to allow for waste.
Visit a stone supplier to choose your stone. Slate flagstones vary in thickness, shape, color and surface texture, and you'll want to see your stone before you buy it.
Prepare the Base
Dig out the bed for the path, removing any sod and digging deep enough to allow for a 2-inch layer of gravel, a 2-inch sand layer and the thickness of the slate.
Spread a 2-inch-thick layer of gravel along the bed, making it as level as you can. Compact the gravel using a plate tamper. If your path won't be subjected to heavy traffic and you don't mind if the slate settles over time, you can leave out the gravel layer and lay the stone directly on a 2- or 3-inch-thick layer of sand.
Lay a 2-inch-thick layer of sand on top of the gravel, and rake it smooth and level.
Lay the Stone
Lay the stone piece by piece into the sand bed, twisting each stone as you press it into the sand to set it firmly. Lay out the irregular stones so that they fit closely together, leaving about a 1/4-inch gap between them and keeping them level with one another. After each stone is set, tap it lightly with a rubber mallet or step on it to set it even more solidly into the sand.
After all the stones are laid, fill the spaces between them with sand or stone dust, and spray the entire path with a light mist of water to settle the sand or dust into the gaps.