I started the process out by creating a 2D drawing of brass knuckles, from an actual pair I had.
Next I needed to design the part that connects to the pizza wheel. I created a few different versions to see which one I liked best. I did a lot of thinking and holding the real item to determine the important ergonomic aspects to consider. You can see that some of the designs have a thumb guard and some do not. Originally I thought this was an important part of the pizza cutter design, after all, that is a blade you are using and you want to guard your fingers from it. But, I found that this just wasn’t necessary when holding the pair of brass knuckles in the appropriate way, your knuckles are shielded and your thumb does not actually rest where I thought it might. So eventually the design on the bottom right was chosen as the one to move forward.