First fold your square in half diagonally. unfold and repeat on the other half.
Step 2: Folding
Now turn the paper over and fold in half whichever way you want. (unfold)
Step 3: Folding
push down in the center of the square and the paper should fold inward. Collapse so the paper forms a triangle
Step 4: Folding
Fold the two bottom corners of the triangle to the top. Repeat on the front and back.
Step 5: Folding
Now the paper should look like a diamond shape. fold the two outer corners in. Repeat on the front and back.
Step 6: Folding
Now the most complicated part, at the bottom of the balloon their are two corners that stick out. Fold these into the two triangle flaps. Repeat on the front and back.
Step 7: Inflating
Now to help form corners when inflating, fold the bottom and top corner to the middle and unfold. After you've done that, open up all four sides. At one end of the balloon there is a hole. Blow into it and it should puff out into a balloon.
Step 8: Using
Now you have a cube shaped balloon. you can fill with water or save it. Since it is paper it will absorb water and get mushy so throw fast.