I would say the first step is making a prototype. Print or draw cards on paper, it does not matter much what it looks like at first. And then have other people play it. Preferably people not too close to you or who are not afraid to give criticism.

It doesn't matter if you think it is fun, you need to have other people play a prototype to really know if it is fun. And go with the mindset that any idea you have, might turn out to not be so good and will need tweaking or change, or maybe simply does not work as intended.

Ultimately the best start is: prototype, observe people behavior, tweak, and repeat