You can't make REAL cards, but you can make Treasure Cards. There are two ways I know of doing this.
The first way is to get either an Accuracy treasure card from the Library or a Strength treasure card also from Library. They only sell for 25 each, so it should be easy to get many of them. Put these in your deck under the Treasure card section. Go into a battle. I suggest Unicorn Way, because the monsters there are the easiest. Please note that whatever treasure card you want to make cannot have 100% Accuracy, or it has to hit something. It cannot be a card from your wand or from your clothes, and though I'm not entirely sure I think it can't be another treasure card. For instance, I could not make a Treasure Card out of Dryad because I only know it from my shoes. And I could not make a treasure card from Pixie (or Fairy, whichever) because it hits no damage and has 100% Accuracy.
To make the cards, delete one card from your hand (press right mouse button when over card) in order to draw a treasure card. If you only put Strength and Accuracy cards in, then that is all you should get. Select the Strength or Accuracy card and then choose the card you wish to turn into a Treasure Card. The two cards should combine and the second card choosen should turn yellow. It is now like a regular treasure card, and you can either use it right away, keep it in your deck for later, or trade it to a friend.
The second way you need a Mutate Card for. These are treasure cards such as Mutate Fire Beetle or Mutate Storm Cat. Do the same process as with Strength and Accuracy cards: make them the only cards in your treasure card deck, draw them, and use them on one of your cards. The only trouble is that it must be the card indicated; you cannot use a Mutate Fire Beetle on a Evil Snowman, because it must be Frost Beetle. As far as I know, it can't be used on cards that you do not own, and I am pretty sure it can't be used on other treasure cards, though not sure.
That was a lot of writing... did it clear things up a little?