Think of something and write it down. Once you have what you think is a workable version, put it down and walk away. No, really. Put it down for at least an hour before coming back to it. What may look like a fabulous idea at first blush may not seem like such a hot idea later. You may see flaws in the idea then that you don't see during the initial excitement. Put it down for at least an hour, a day or more is even better. (I suggest writing on scrap paper rather than sitting at the computer to type. Type in step two and three)
You need to do a quick check to make sure you’ve told us all the important data on the card. This applies to all card types.
Who can play it?
Are there any requirements?
Does the card have an ongoing effect or does end it end after a specific amount of time? When does it end?
Does is have all the right keywords on it?
Did you fill in ALL the relevant data? If a space is meant to be blank, STATE in the description it's blank so it's clear it wasn't overlooked.
Balance. Look at the card. How powerful is it? More powerful cards should be harder to play. Weaker cards should be easier to play. Some of this will be fixed in playtesting, but it's easier to make a good balanced card GREAT than it is to make a broken card good.
Show the card to someone else. They will point out any glaring errors you missed and better be able to see problems than you will. It is afterall, your card, and what may seem plainly evident to you, in your thoughts, may not have been accurately conveyed in your words.
Submit the card!
Other things to keep in mind with design:
Keywords are AWESOME. Use them.
If you're submitting a whole bunch of cards at once read over them and if you find you're saying almost the same thing over and over and over again in slightly different variations, consider making that thing a keyword instead.
For example, if you have several cards that are predators you may have described it slightly differently each time you described the effect. "The predators eats its target" "The predator hunts its prey" "the predators exhausts the target creature", etc. Make it a keyword instead! Condense those similar sentences into one keyword "Predator" and replace the text with the Keyword. Then everytime it appears on ANY card, it always works exactly the same.
Keywords also make it easier for younger kids to play since they may be able to recognize the Keyword even when they couldn't read the whole sentence without help.
Keywords also make it easier to play with cards in translation. Then only the rules for the Keyword itself need to be translated, not each individual card. Predator does the same thing in English and French. The French speaker just needs to be able to recognize the English word and can read the meaning of that keyword in the French rules. (or in Chinese or Russian or Inuit or whatever language the core rules are translated into)
Keywords increase accessibility.