Measure the width of the border you want to create, from the edge of the yard into the yard, with a measuring tape. Drive a garden stake into the soil at that point to mark where the border will go. For example, if you want a 6-inch border around your yard, measure 6 inches into the yard from the edge and drive the stake.
Move over 3 to 4 feet and repeat the process of measuring the width of the border and driving a marker stake into the yard. Continue marking out the border until you have gone around your entire yard.
Run a garden tiller at a depth of 3 inches along the edge of the border farthest in the yard, following the stakes. Pull the stakes up as you reach them with the tiller.
Use a shovel to widen the trench created by the garden tiller, digging out the remainder of the border so the entire border is 3 inches lower than the yard level.
Fill the depressed border with a 3-inch layer of mulch. The depression will prevent the mulch border from sliding into the yard after rains. Use whatever inorganic or organic mulch you choose.
Rake over the mulch border to smooth the mulch.