1. Make a practice card on printer paper if you're making a card for the first time - it will let you practice and give you something to trace with.
2. Pencil is your friend! It helps you avoid spelling errors and let's you decide what "font" to use, so to speak.
3. Buy multiples of the same shade/pattern of paper. That way if you screw up, you have backup, and if not, you just have an excuse to make more cards.