Many times I get asked "Did you build your droid from a kit?"
And my answer is always "No."
Most parts are purchased separately from different approved vendors. One must do their homework in terms of what goes where, how to attach parts, what type of paint to use and how the electronics are hooked up. There is more than one way to do all of these things. Some work better than others.
I highly recommend logging in to Astromech.net. Here you will find an astounding amount of information regarding every possible subject related to droid building. You can spend months on this website absorbing all of the information available. And there is a lot of information.