Natural stone is a great way to landscape a hillside. As a natural stone, flagstone offers a smooth texture that makes it a good choice for a walkway. The biggest challenge you will face is the slope of the hill. However, regardless of the slope, keep the grade of the area you dig for the walkway as flat as possible.
Place several flagstones on the grass to determine the desired width of your walkway. Because the shape of flagstone varies, turn the flagstone so that the shapes complement each other.
Spray the grass where the walkway will be built with marking paint. Keep the paint inside the area to be dug so it will disappear when you dig out the area of the walkway.
Dig the shape of the walkway with a sharp pointed shovel. Dig down about 6 to 8 inches to allow for the rock base, which is the foundation of the walkway and allows for drainage. This also is a good time to install a separate border for the flagstone walkway. The border can be made using concrete or landscape timbers or even a different type of stone. If the slope of the hill is too steep to keep rock and sand in place, dig the steps out as needed. For more on flagstone steps, see "Constructing Flagstone Steps in the Landscape" in the Reference section of this article.
Lay 1/2-inch crushed rock, making the layer about 3 inches deep. Roll out a layer of landscape fabric to prevent unwanted weeds or grass from growing between the flagstones. Pour sand over the landscape fabric, about 2 inches deep, to create a soft, yet firm foundation for the flagstones. Rake the sand to produce a smooth surface.
Place the flagstone in the walkway as if you were putting together a large jigsaw puzzle. Because the stone is placed on the sand, it is possible to keep the surface of the stones even. Simply remove or add sand under a flagstone to accomplish this. Once a piece is set in place, use a rubber hammer to 'seat' the stone in the sand. Keep the gaps between the stone almost equal and fill them with sand. Sweep off the excess.