This is the sole of the D7. I chopped this off so you could have a sample to play with. I'm new to designing as well and figured this might help. I can't wait to see what you create. Use the tag D7tread!

https://blog.thangs.com/take-a-step-in-a-new-direction-design-a-tread-for-the-d7-3d-printed-shoe-c502414a6957