Decide how wide you want the base of the retaining wall to be and dig down with a shovel within the perimeter. You need about 6 inches of space for the gravel layer and another few inches for the layer of sand that goes on top of the gravel. This provides drainage and compacting support for the weight of the retaining wall.
Shovel in some of the gravel and tamp it down so that it is compacted into the soil. A couple of inches is sufficient for the first layer then add another layer on top of that and repeat the tamping process. Once you finish with the gravel, you need to compact 2 or 3 inches of sand on top of that. Ensure that the finish layer of compacted sand is sufficiently tamped down so it doesn’t fall between the gravel.
Layer the gravel and sand with some of the tumbled stones. Arrange them in a random pattern and butt each stone up against its neighbor. Work your way down the length of the wall then work back in the opposite direction with another layer on top of the first. Turn stones in any direction you want to get them to fit together and work your way up until you are finished.