Details in "Building Montessori Binomial Cube at 29 months" montessorifromtheheart.com/my-blog/2016/04/montessori-binomial-cube.html
First, the child will take all the cubes out of the wooden box and group and sort them by color on a table or a mat. Then, starting with the largest single-color cube (red), the child will complete the level by placing the adjoining cubes so that the same colors "touch". Once the level is completed, the child will slide it to the wooden box. Similarly, the child will proceed with the second largest single-color cube (blue). The control of error: the child runs his or her finger across the top of the cubes to see if all the cubes are the same level.