To make an amazing birthday card for your friend, you may need to have some materials like card-stock, construction paper, stationary, glue, stickers and coloring utensils. Take a piece of construction paper and fold it into quarters or at the middle, depending on the size of card you need to have. Think of a good theme, depending on your friend's interests and characters. You can then draw up something that she likes and color it well. You can learn more about this from http://www.scrapbooksetc.com/crafts/cards/.
I always make custom cards for a special situation for my friends. You need colored pencils, any construction paper, and a creative mind. You can add stickers an other stuff but I write like happy birthday in big bubble letters or whatever and you can decorate flowers around it, but inside the card, write how important it is that you aren't writing the card JUST because it is their birthday, you're writing a card because it is your best friend. (You can add BEST FRIEND'S birthday.