Note* Sorry for all the blurring. I didn't want anyone calling my office :)
Step 1: Eyeball Measuring
You'll want to ensure that you have an equal amount on either side, though it is not critical to measure.
The side of the business card that you want to show on your cube should be face down on the table at this point.
Step 2: The Fold
Press down on the top card and fold the bottom card up.
Make sure that you do not crease the folds as they will become too flimsy and won't hold the cube together.
Step 3: Repeat!
Your folds should be 90 degrees or less. If they are too wide, the cube will not hold together nicely. I find it best to get the folds to 90, then make them a bit smaller.
Step 4: Assembly
Note in the picture how the cards fit together. Always have the little flaps inside the cube.
Step 5: Display!
Now it's time to show off your work! I was going to say hard work, but that wasn't hard at all!
Note* For some added support, you can put a small piece of scotch tape on each seam. It is virtually invisible and your cube won't fall apart if you're playing with it.