For instance, I got an Evo 3D. I wanted an extended battery to go with it. The only one with a case available is by Sedio. It's super expensive, and I don't like the design. I went about finding a cheaper one; of course, the cheaper extended batteries don't have cases.
Since, I have a FDM printer lying around, I went about creating a custom case for it.
This instructable will teach you how to model the case to fit the phone/any device.
Step 1: Tools and material
* Vector based graphics software (autoCad, Adobe Illustrator, solidworks, what have you)
* 3D modelling softare (Solidworks, autoCad, 3D studio max, etc)
Step 2: Preparing reference photos
The way I accomplish this (since I don't have a 3D scanner) is to create reference photos of the object's projections. I used a scanner to get exact orthogonal views, however, if you're handy with a camera, you can use a camera instead. I suggest trying to fix the focal point of the image to be at the same spot for all views to keep it consistent.
Resize the photos outside of the vector editor if you want, I did it in Adobe Illustrator.
To get the sizing correct, choose as many reference features in the images as possible (the size of the camera button, the size of the camera lens, the length of the body, etc). Use calipers and rules to measure those features, and adjust the scale of the image accordingly.