Learning CAD
If you've never touched CAD software before, then you should start with the basics. Many use Fusion 360 with a personal license as it is free to use and does everything needed to design a DIY steering wheel.
A very easy to follow and well-explained series is by Arnold Rowntree, whilst it may seem irrelevant at first, understanding the fundamentals of CAD is vital to creating a DIY wheel - be sure to watch all 16 parts.
Once you know the basics of using CAD, you should check this out to get an understanding of the components. This will help keep your files clean and easy to edit.
