I've created a case for my Raspberry Pi in Tinkercad and would like to 3d print it. However, I don't know how to create supports! I tried looking it up, but I got a model that I don't know how to import, and some best practices for supports. So how should I create them using Tinkercad?
If you're using the term "supports" to mean the extra material that the 3D printer needs to allow material to be deposited in "mid-air," your slicer will have a setting that permits this. Knowing which slicer is going to be used would enable someone to advise you directly of the location of the settings.
If you are going to be using a 3D printing service, you need not address those types of supports. The printing service will have qualified operators who will select the correct settings for your specific model.