The unit process goes something like this:
- Concept. What you want the unit to do, and what you want the unit to look like while doing it. There's not really a tutorial for this.
- Model. Modelling is a very general thing; for tutorials you should just search the web in general for how to model in your application of choice.
- UV map. There's a couple UV mapping tutorials stickied in the Art & Modelling forum.
- Texture(s). Texturing is a mixed bag; there's a few tutorials for specific techniques both on these forums and on the web overall, but mostly you have to experiment.
- Upspring. Upspring is the program that you use to turn your model into a Spring-ready .s3o. Again, there's a few tutorials on the forums.
- Scripting. There's some tutorials on COB out there, but if you have no programming experience, it may be difficult.
- .fbi/.lua. The file that defines the unit's statistics. Generally best to copy an existing one, rename it, and change what you need.
I'll try to do an end-to-end tutorial for my next unit, but it will be a while until then, so don't hold your breath.
As for the order I learned things: I had programming experience before coming to Spring, so scripting was the first thing I learned. Then I started hacking pieces off .3dos and sticking them together into new models. Then I started making .3do models from scratch, using existing textures. Finally, I started making .s3os, doing both model and textures.