You can use sand to help level out the cement blocks. Just get the measurements between the 4x4s on the bottom to make sure the blocks are spaced correctly. Use a 2x4 and lay it across the blocks to see which ones need to be raised or lowered etc. It's really not too hard, took me an hour to set mine up

I just built a 10x12 lean-to style shed this summer (I drew my own plans with several variations and found the monoslope roof without any overhang to be the cheapest option available to me). My site is reasonably level naturally. I dug down about 6 inches and had a clean pea gravel fill dumped in the hole. I leveled out the gravel and then made a couple passes over the whole area with a vibrator to get some initial compaction. Then I filled in the low spots and did a bunch of passes with the vibrator to compact the fill as much as reasonably possible. This would not be considered a great foundation in NY, but for a shed I do not care if there is some slight settlement. For such a small structure, I don't expect there to be much settlement anyways as the whole area is over consolidated clay.