First, I searched for an image until I found a map I liked, this one happened to be created by gogots. I then used photoshop to cut out the floor of the image leaving just the walls usIng the polygonal lasso tool. This is important, for if the image has to many polygons tabletop simulator will not cut out the image.

Once I had only the walls remaining I saved the image as a PNG and uploaded it to the game as a custom token, setting the tokens Thickness scale to it's maximum. Then I scaled up the token until it is the right size and set the coordinates of X and Z using the gizmo tool to 0.

I then copy and paste more layers on top of those tokens to make the wall higher. Do not stretch the tokens using the gizmo tool, as it will revert to its original size when you reload the map. Make sure that the area you cut out leads outside of the image, as Tabletop Simulator will not cut out tokens that are transparent in the center.

please share how you made the outside walls... it looks awesome... hopefully you can post a video. I want my battle mats to look like this !!