Excavate for the Border
A sturdy gravel patio should be at least 4" thick, but to avoid having to step up onto the patio, excavate before starting the patio. To begin, mark the border location and dig a trench 12" wide and about 2" deep to serve as the foundation for the slate border. By digging the trench first – before excavating for the gravel – there's no need to build a form to shape the concrete. The soil does the job.
Note: Use a heavy-duty rototiller to break up and remove harder soil, setting the soil aside to create a planting mound. When the bulk of the soil is out, level out the surface with square shovels. If the area has poor drainage, add drainage pipe or pitch the soil to drain as desired.