This is only DIY advice, but when we recently made a flower bed on a slope next to a foundation, we made a level string line, started at the bottom of the slope, leveling as we went, and worked our way up the slope. We drilled holes in the landscape timbers -- 3 for 8 footers -- and pounded 18-24 inch rebars through for support. We also used 8 inch landscape nails to link the supports together. For the slope side, if it got above 10 inches or so, we got a long drill and drill adjoining holes and pounded more rebar through to keep if from tilting. We put gravel -- an inch or two, then topsoil and peat moss mix in the bed for the flowers. Added a drip hose with a time through the beds as well. Good luck.