Solid concrete blocks can be made with the right molds and a little know-how. Hollow concrete blocks, however, require a considerable amount of finesse to come off correctly. The best method of creating hollow concrete blocks at home involves using inflatable balls as the molds. That creates a spherical pattern that can be used as flower pots and gardening affectations, as well as serving more practical duties. Hollow blocks made in other shapes require much more specialized equipment.
Inflate a beach ball and use a felt pen to draw a circle around the top, with the air valve in the center. This will be the opening of your concrete block.
Apply water-based concrete-releasing agents to the entire surface of the ball.
Wrap strips of hardware cloth around the surface of the ball, leaving the area inside the drawn circle clear. Be sure the strips remain as evenly spaced as possible: Multiple layers on top of one another will create an uneven surface, which makes it trickier to get the concrete sphere right.
Place a layer of wet concrete around the surface of the ball. The layer should be a uniform 1/2 inch thick around the entire surface. The area inside the drawn circle should be kept completely free of concrete mix.
Allow the concrete to harden for at least 24 hours.
Deflate the ball and carefully remove it from the concrete.
Add additional layers of concrete over the first until the block is of the desired thickness. You may wish to do this in stages, adding a little concrete at a time instead of placing it all on at once.