supplies:
recycled sheet of paper
scissors

(1) fold & cut off extra
fold one corner across to form a right triangle. Cut off the rectangle area and unfold. You made a square.

(2) fold & unfold again
fold the opposite corners together, then unfold again.

(3) flip, fold across & unfold
turn the paper over and fold the bottom up. Unfold then flip the paper back over.

(4) push in sides
while pushing the left and right sides in fold the bottom up to make an upside down triangle.

(5) top corners down
take the 2 top corners and fold them down to match up with the point at the bottom. Don't fold the corners on the back ... yet.

(6) flip and repeat
turn the paper over and do the same on the other side.

(7) fold outside corners in
now, take both outside corners and fold them in to point at the center.

(8) flip and repeat
turn the paper over and ... (you figure it out).

(9) bottom corners up
take the points at the bottom and fold them up as shown.

(10) tuck in folded corners
take the two points you just folded and tuck them inside the triangles made in step 7. Not under, INSIDE!

(11) flip and ... repeat!
same thing again, fold up then tuck in.

(12) crease top and bottom
this step isn't required but it makes a better looking cube.

(13) inflate
separate the sides and blow into the hole at the top. Avoid spitting and hyperventilating.

(14) cuberific!

(15) who wants a bath?
fill with water through the hole at the top.