Choose a mold. Make concrete forms from boards, metal tubs, plastic or anything that will keep its shape. Two inch-by-four inch boards nailed together in a square and nailed to a piece of plywood will make a tabletop or bench form. Nail smaller sections together for table or bench legs.
Mix concrete or mortar according to the package directions in a wheelbarrow or large tub with a hoe. Three fourths of the concrete used is ready or premixed. For a more fine texture a less course aggregate or sand should be used. Mortar is mixed with sand and water for a smooth texture. Quick concrete has many small stones for a rougher finish. The finished mix should not be so runny that it oozes out of the mold. It will be the consistency of pudding.
Pour the concrete into the mold. For a bench or other large piece of furniture add lengths of steel reinforcement called rebar. Lay the rebar in the wet concrete and make sure it is completely buried. If the bottom of the mold is going to be the top of the piece of furniture don't let it sink to the bottom. After the concrete has set a bit, lay the rebar on the top and smooth more wet concrete over the bars.
Add mosaic pieces made from broken pottery or glass or other decorative items to the wet concrete at this time.
Allow the concrete to cure and harden for 48 hours.
Remove the hardened furniture pieces from the mold. The two-by-four boards can be pried loose and the mold taken apart. Other molds will need to be turned over and the mold taken off the piece. The top of a 55 gallon drum makes a good table mold. Turned upside down the concrete piece should fall out.
Assemble benches, chairs or tables from the concrete pieces. Use concrete adhesive or more mortar to keep the table or bench top on the legs. A bench will need a seat and two legs. A table will need a top and a pedestal or legs.