I was so proud of the fact that I cleaned off my rototiller and started defining the edges of my new miniature garden. The soil is better than I thought, just compacted. So I got about a third of the way through when the sky started rumbling.
Then the sky opened up and at least an inch of heavy rain (and maybe two) fell and washed it all away. Clearly I need to work on draining the excess water away from this area before I plant. Sigh.
The Wonder of Miniature Worlds 
I know exactly how you feel. It’s so hard to get the soil just right when we are all experiencing such heavy rain. I have clayey soil so I mix lots of bark amd perlite in with it which helps.
Put a small stone wall along the bottom edge, to keep the soil from migrating when it rains.
I was thinking of that (obviously too late for this storm) so I need to search out some rocks that are the correct scale but also big enough to actually keep the soil from flowing away. Probably the first thing I should have done, even before tilling the soil.
You can also use 1×6 boards instead of stones. Just pound in a 2″x2″ stake every 3 or 4′ then screw the board to it. Stones are free if you have a creek or river near by. Either way… enjoy the adventure.
I also have some super heavy cinder blocks that could use a permanent place in the garden.