There are many topics to cover, including:
CAD design / Mechanical design / Product design
Strength of materials / Finite Element methods
Measurements / Geometric Tolerances
Reverse Engineering / CMM / Photogrammetry
Additive and Subtractive CAM methods
Marketing / Sales / Contract optimization
Logistics / Supply Chain management
Industrial regulations and compliance
Project management and stakeholder communications
Online commerce / Mass customized goods
And so on.
It's a lot of stuff, and you'll probably have to learn based on individual project needs
You'll also need to build a network of resources you can draw on to reduce project risks stemming from your own lack of experience, meaning you will need a focus on communication, and management.
You would not be blamed if you felt a need to go back to school for some of this. However, the school of contracting is also rather effective, assuming you are an autodidact and can surround yourself with the resources you need to ensure project success.
Good luck!