@xBRIANx No, questions are good, that’s how you learn. First, the slope of the hill I built my boxes on is about 20 degrees (I’m taking an educated guess at that number). If your slope is more severe, you may have to stack an additional 2×10 on top to ensure that you don’t go below ground level on the top slope side of the box.
My 2×2s I used for the corner supports are pounded in to a depth of 10 inches and flush with the top of the box. Also, I supported the middle of my boxes with a 2×4 affixed in the center/top of the boxes (also flush with the top of the box) to keep them from bowing under strain. Use 3 inch coarse deck screws to keep everything together. Make sure that you screw the outside of the box together in addition to affixing it from the inside corners with the 2×2s.
I used this same design on my garden last year and I no problems at all using treated lumber. Cedar is prohibitively expensive and you will be hard pressed to find the dimensions you need to build a stable box to hold all the soil.
Once you get them built, the next step is making sure you have good rich soil to plant in. If you’d like, I can give you my soil “recipe” that will give you incredible results (I damn near guarantee it!!). Hope this helps.