Step 1: Idea.
Here are some examples:
Subtracting health (Pokemon or Bakugan)
Three strikes your out (Redakai)
Majority (Instructables card game)
Be sure to chose a theme too. I chose a medieval card game with castles and dragons. Then after i chose the theme i chose a title Castle Quest was mine but you can come up with your own. Make sure the title portrays the game too.
Step 2: Design.
If you are making a card game by hand you might want to use card-stock paper so that the cards aren't see-through. To make cards the same size as mine you need to fold the paper hamburger style and then unfold. After that fold the paper hot dog style. Then turn the paper to landscape and fold the left and right side to the center. you should have 8 little boxes. Then you can color the cards however you want really.
You can also use things like ccg maker and bighugelabs. But i wanted my cards to have their own style.