I talked to a guy at work about the leveling and he suggested gravel. That sounded good. I headed for OSH at lunch time and asked an actually-knowledgable person about it. She said these sheds don't really need gravel, they don't even need to be absolutely level. She suggested that I get a wide rake or make a tool from 2X4s - a T instrument that I could drag across the dirt to determine what needs to be filled. She said I could even use potting soil.
So that is more or less what I did. I used potting soil to fill in holes, took a rake and made it reasonably even. Then dropped the floor down on it and started assembling.
It's a lot easier than real building. A lot of the parts snap together. I had some touchy moments but got past them.The wind was blowing all the time so pieces would pull loose. The parts each depend for their integrity on each other, so until I had the shell done it was vulnerable. Finally it was time for the roofing. I had a bit of a time with the end pieces and I was unable to insert the plastic connector in one corner. Couldn't figure out why. But I was able to go on, get a beam across the center and then one perpendicular to that one, and then I started on the roofing panels. There are four of these. They slip under the top cross beam and hook under, then lie down on the end and middle supports. Then there are little clips that are supposed to snap them to the center beam. Only I was not able to get them to snap. I felt by that time that I no longer had the strength or the will to continue so I came inside. I feel a little frustrated that I did not finish it but I feel amazed, still, at getting as far as I did.