Step 1: Downloading Template
Step 2: Customising Design
For this Instructable I will be using Sketchup. Bring the STL file in to your chosen software. Sketchup does not allow the import of STL files so I used Netfabb to convert the STL to 3DS which Sketchup allows. Once you have the file you can start customizing the design. You can draw using the various tools on to the surface and use the Push Pull tool to add or remove part of the back to create your design. If you would like some inspiration please visit my Thingiverse Profile where I have various case designs.
You can design a case with a repeating pattern like my Honeycomb design or circles or an interlocking shape. You could also produce a case with your name or even a 3D location map. There is software available where you can get a location and it will render it in 3D. You could even produce a case with moving parts like my Abacus case which has 100 moving parts.