I am planning to design a couple of phone cases which contain features other phone cases don't have.

My plan of action is like this:

• Design phone cases on computer
• Send emails around to companies that manufacture and sell phone cases showing my designs and asking for a certain price (like 10% sales on the phone cases)

This is first time I am doing something like this. Is the approach I am planning to do is correct in trying to get a company interested in my phones cases designs?

(If this is wrong place to ask this question then can you guide me to the correct Stack Exchange site please)