I bought a Rubbermaid tool shed from Home Depot and made the mistake of trying to build it directly on uneven ground. It said in the instructions that it could be assembled on "reasonably" flat ground but this wasn't reasonable. I went to Home Depot and got a bunch of Quikrete crack resistant concrete and poured a slab in less than 4 hours of total work time and it didn't cost me more than $120 in materials.
Quikrete Crack Resistant Concrete Available at Home Depot
STEP 1: FAIL!
I learned not to take shortcuts from this project. I spent a few hours trying to force the pieces together but the slope of the ground caused the flexible plastic to bend out.
STEP 2: Make a frame.
I screwed together 2x4s and 1x4s to make a perimeter frame for the slab. I added a middle 2x4 so that I could pour the slab in 2 pours. This isn't necessary but we were mixing the concrete by hand and this broke it up into two smaller projects. I drove wooden stakes into the ground and then screwed the frame to the stakes using a level to make sure the frame was leveled.
STEP 3: Add gravel.
I spread out 3 inches of Quikrete 3/4" gravel and packed it down with a tamper.
STEP 4: Mix and pour the concrete.
We mixed the Quikrete Crack Resistant Concrete 1 bag at a time and spread it in the frame.
STEP 5: Screed.
I used a straight board to screed the top of the concrete and make the slab nice and level.
STEP 6: Pour the second half.
We waited 24 hours and then removed the center 2x4 and mixed and poured the 2nd half and screed the top.
STEP 7: Fix the shed.
With a nice flat slab we were able to fix the shed in just a few hours!