Measure a 9-by-8 1/2-foot rectangular plot at the location where you want your planter box. Mark the perimeter with marking paint.
Till the dirt in the rectangle until it's well broken up to a depth of several inches.
Dig a shallow trench -- about 1 inch is sufficient -- around the perimeter of the plot. Place the dirt you excavate into the middle of the plot.
Lay two railroad ties along the long edge of the rectangle. Place a level onto the ties to check the slope. Move the ties and remove soil from beneath them as necessary to level the ties.
Place the other two ties perpendicular to the first two ties to complete the rectangle. Use the level to ensure they're evenly laid, removing dirt as necessary to do so.
Fill in the planter box with topsoil and rake it smooth to within 1 inch of the top edge of the ties.