Hi there, I'm currently working on an isometric game, took me a while to figure out using tilemaps with it. I basically create the "always behind" game layout (floors and walls) in Photoshop using a isometric grid image as a background and turning on "show grid" set at 64px with 32px intervals.
Then the tricky bit which takes a lot of trial and error is selecting a vertical slice say 128px wide that can repeat. It's probably best if I send you examples but I'm away from my laptop at the mo. Feel free to message.