The hard part is building up the downside of the bed and cutting into the upside of the slope so that the bed is level.
I started by putting down a few bricks to support the wood on the downside slope.
Then I dug narrow trenches into the upside where the boards could nestle in.
I used a level to determine when the boards were at the correct angle. My soil is clay-like and full of rocks and roots of all sizes. I wedged the clumps of clay, rocks and sod that I dug up underneath the downside boards and put bigger rocks against the downslope planks to prevent erosion. If the soil does end up washing away when it rains, I may have to invest in more bricks. (If you have done this and have any tips for preventing that, please let me know!)