Stack all the eggplant slices on your cutting board. It's helpful to line them up by size. Cut them into sections that are roughly square. The wide end of the eggplant will be cut slightly larger than the small end. That will give us squares that can be stacked easily without fear of them toppling over.
You can rearrange the slices at this point if you like, so you know how many stacks you are making. Depending on the size of your eggplant, you may have more or less slices than I do. Select the biggest squares for the base, and stack them in reducing size so they are slightly pyramid shaped. If you have a lot of slices, you can make 5 (or even 6?) stacks.