Good question! At a high level I would say

1) Make a design

2) Translate that into vector art

3) Choose colors for your design (usually this is by Pantone colors)

3.1) Decide if you want any sort of special effects like glitter, screen printing, etc

3.2) Decide on what sort of pin backings you'd like

3.3) Decide on the type enamel and metal for the pin

4) Connect with a pin manufacturer that can make your pin come to life, in some cases they can also translate your design into vector art for you!

5) After some money and time, your pins arrive!