Several layers of fabric are sewn together, and then the designs are cut out from the top layers to expose the different layers beneath. It's obviously a very detailed and time-consuming process. The more colors (i.e. layers) the greater the level of difficulty.
The molas are now hanging in our entryway and I love the burst of color they add!
These pieces are beautiful, especially when you look closely and appreciate the fine details.
For more information, see Wikipedia.