Cube is signed exactly like block, so it looks like you are outlining the sides of a block. You start by taking your open hands parallel and a few inches apart. Orient your hands to outline the front & back, then the left and right side, then the top and bottom of the block. The sign for box is very similar, you just keep your hands further apart because the box is bigger.
Teach your baby or toddler the cube sign when they show curiosity for differently shaped objects around them. Square blocks are the perfect examples of cubes, so you can start there, then progress to boxes that are perfectly shaped as cubes, ice cubes, plushie dice, and so on.
Teach your toddler this sign when they have already learned the concept of 2-dimensional shapes (circle, square, triangle, rectangle) and are ready to learn their 3-dimensional counterparts (sphere, cube, pyramid or tetrahedron, etc.) Then be sure to relate these shapes with their everyday object equivalents.