Building a concrete hot tub requires a general knowledge of your ground plan, slope, geology, weather conditions, home value, landscaping, and a variety of other building aspects. You can't just dig a hole and pour in concrete. Once you've chosen the area to build your concrete hot tub, measure the dimensions so you know how much concrete to order. Be sure to pull any permits and obtain any required permission from the city you live in.
Map out the hot tub area and dig a hole approximately 5 feet deep. You can rent a backhoe, but you probably only need shovels and a few laborers.
Set up any plumbing that needs to be installed prior to placing any structural supports in the hole. It is best to call a plumber for this part of the job.
Place 3-inch concrete blocks along the bottom of the hole. The blocks keep the rebar off the ground.
Lay out rebar on the bottom of the hot tub in a criss-cross pattern that extends up the sidewalls creating a bowl shape. Place the rebar 6 inches apart.
Tie the rebar together at each crossing point with a heavy duty rebar wire. Make each piece 6 inches long, and use pliers to tightly wrap the wire around the rebar.
Pour concrete into the hole, making sure that the rebar on the sidewalls does not touch the earth. Rebar that is exposed to dirt will corrode and swell, eventually cracking the concrete. Trowel the concrete until it is smooth and even.
Fill the hole with Gunite, a layer of waterproof plaster, after the concrete cures. Spray it through a gun at high pressure on top of the concrete, and trowel all the sides and bottoms until it is smooth. Start at one side of the hot tub and trowel to the far end.