The 3D design, or CAD, is just a matter of sitting down, loading a free program off the interweb, and then looking for YouTube tutorials. There is no way that you would just 'figure it out', these programs are not simple beasts, they confuse the hell out of mere mortals. That said, it's a building block process. You learn the simple things, and then use two simple things to create a slightly more complicated thing, create a few slightly complicated things and then use those to create a quite complicated thing, and so on.
It took me a couple of years to realise what I could do with Openscad, and how to do it to get the parts I wanted, but I have reached the point where I can pretty much design whatever I want. With hindsight I should have gone with something like Blender, as it is far more capable, but I am now in the Openscad groove so I stick with it.
The key thing to remember is that there is a wealth of information on the web, and You Tube is a godsend, it's a matter of finding a tutorial that you equate with, some are diabolical and assume prior knowledge, whereas anyone starting out knows nothing at all.
