Summarize this article in one sentence.
This is the card that will be the foundation of your project. The color you choose will be featured prominently in your finished product, so keep that in mind when choosing your base card.  Craft stores sell blank, plain cards for this purpose. These are already folded and creased, making them easiest and fastest to use. The blank cards come in boundless varieties. If you really love making handmade cards, definitely pick up a pack of blank cards (or cardstock) next time you’re at the craft store. That way it’s on hand and ready for you at a moment’s notice! You can use any kind of paper you want, but the best choice is a durable paper that has some weight to it. The most popular choice is cardstock, which is a nice heavy paper made specifically for card making. All craft stores carry cardstock and the variety is usually vast. Your base card choice can be as simple as plain white paper! Paper that is any color of the rainbow and patterned paper are both great options as well. If applicable, consider upcycling your child's artwork for the base card, or even your own artwork! This is an even better idea if you need to make the card at the last minute and don't have a lot of craft supplies on hand at the time. If you’re using a full piece of paper rather than a pre-folded card, you’ll need to go ahead and fold the paper so that it will be in card form. The best looking cards have a nice, crisp crease. This is something you can do very easily with items you already have at home.  With your paper horizontal in front of you, take a ruler and measure the horizontal center of the card in two places – near the top and near the bottom. Mark the center lightly with a pencil dot in both places, and then use the ruler to connect those dots with a straight line, from the bottom to the top of the page. Use the pencil with a very light touch. If you are crafty and already have a bone folder, this is the tool you will use to score the card. However, you can use a dull butter knife and get the same results. Scoring the card will ensure a nice, crisp fold.  Line your ruler up next to the center line you’ve just drawn and take the butter knife to score the card along that line. Scoring means to push firmly into the paper so that a visible indention occurs. Be sure not to push too hard! Once scored along the drawn line, lightly erase your pencil marks. Do this by carefully folding the paper along the scored line. Once folded, use the bone folder or some other flat tool to flatten the fold very crisply.  Using the spine of a book would be a simple solution if you don’t have a bone folder. You should now have a crisply folded piece of paper that looks almost exactly like a store bought card!

Summary:
Choose a base card. Prep the paper for folding. Score the card. Fold the card.