Disclaimer: I took the pictures with a Nintendo DSi. Thank you for making those, Nintendo!
Another Disclaimer: I do not own the design for this cube. Credit for the design goes to Shuzo Fujimoto.
Step 1: Cut the Card (Skip This Step If You Are Starting With a Square)
Okie dokie, so you have a note card most likely if you are doing this step. Now fold the card diagonally as shown in picture 2. Crease using your finger nail(you will be creasing with your finger nail this whole time).Now look at picture 4. Crease where the box tells you to. Now tear on the crease. If you need help as to where to tear, look at picture 5(but don't tear with just two fingers! Use both hands!). Picture 6 shows the result if you did it right.
Step 2: The Tedious Part
If you started at this part, look at picture 1. Crease the square in half diagonally(with your finger nail). Look at picture 2. Now, crease the square in fourths horizontally and vertically as shown(blatently with your finger nail). Picture 3 shows the result if you creased correctly. Now crease diagonally in the direction equal to that of the crease made in picture 1. If you need help, look at pictures 4, 5, and 6. Picture 7 shows the end result of this step.