Oddly, the thing I found most difficult about this was using Pyxel Edit to actually put the tilesets together. Not that PE was a problem, *I* was a problem. I ended up overwriting some tiles here and there, then creating duplicate tiles as I was moving between the editor and the tile set itself. But, the lesson is learned.
For anyone interested in the process, I started with a “dirt” image from some other tilesets out there, edited it a bit in Photoshop CS4, brought it down to 64px x 64px (since that’s the “unit” size I’m using in Unity) and used that as the base. The darker dirt color was that base. For the lighter dirt, I lightened the tiles or selections from the tiles by +30. For the cave walls I darkened the selections by -70, then ran Photoshop’s Sponge filter with settings of (0, 0, 8).
Beyond the base full dark dirt tile, I mostly worked in a 3×3 grid with a feathered mask that ran from 32px in and over to 160px back and down, making a sort of rounded, feathered area that blended from a midpoint 32 pixels into the edges. This helps ensure that all tiles will fit together nicely and look decent.
For the diagonal tiles, I overlaid opposing corners, used the polygon lasso tool to cut across the opposite diagonal, merged the layers down and BOOM! There’s a diagonal tile.
The sections at the very top center and bottom center are where the three types of terrain blend together. I’m fairly certain that I’m missing a few combinations, but this should be pretty good to start.
For anyone looking to work in tilesets and the like, Pyxel Edit is a GREAT tool. And I think it costs about US$9, so it won’t break the bank.