Below find help to avoid the spa from freezing and what to do to winterize your hot tub.
How do I protect the Spa from Freezing?
Normally the spa, when operating, will not freeze unless you turn it off.
If the power goes out any length of time or the unit fails, you need to act. Although water temperature will maintain for awhile (don't keep checking and allow heat to escape) you need to be proactive.
If you know you will not have your spa repaired quickly, you will need to either drain it (above) or protect it. In most climates, simply putting a heat lamp or even a shop light in the equipment area can provide enough heat from the bulb to keep things unfrozen. But the critical parts are the plumbing. For spas with a lot of foam, this too can be a blessing and curse. But making sure whatever method you are using, nothing hot is touching any equipment or wet area or foam. Do not place any electrical object in water. If you drain the spa you can also put the light/heater in the water area of the spa to warm from there into the pipes. But is short, do not depend on the draining to keep you safe. I have found spas that the owner "thought" the water was drained well but wasn't and bottom fittings and pipes were burst. This can be a major job. By assuring the spa/water is kept warm by either heat source in the equipment area or in a dry spa will be money well spent.
Another effective way to protect your spa from freezing if the spa shuts down is "pipe heater tape". This is a common pipe tape found at most hardware stores. They are designed to wrap around home water pipes in cold climates. Once you wrap them around the pipes you plug the tape in. It has a sensor and ONLY comes one when the pipe gets cold.
Wrap some of the pipes with one or two heater tapes, especially around the pump and heater. Then plug the heater in an outlet that isn't connected to the spa. That way if the spa caused the breaker to trip, the heater tape is ready to save the day. Of course you would want to make sure any wiring exposed to the elements were protected from the elements.
How to Drain Your Spa:
Your spa likely was designed to drain well when you use the built-in drain. But several areas may not drain well so you need to give them attention.
First, remove the left over water inside the spa's footwell area after draining.
Second, depending on your spa, you have 1-3 pumps to be concern about. On each pump loosen the front fitting where the pump mates with the spa plumbing. On some pumps you will see a knob drain valve below the front fitting. This is there to allow remaining water to drain. Loosen or remove.
On smaller circulation pumps you have to pull the hose off the front of the pump. There will still be water in many pumps so if the weather is cold in your area, a wet vacuum can be used to suck the remaining water out of the pump.
Third, the heater. Loosen one or both fittings to the heater to allow the water to drain. If you find no water dripping out when you loosen, shake the connection to break a possible seal to allow water to drain.
Many homeowners will take their wet vacuum to each jet and suction fitting on the water side and suck any remaining water out of the piping. This or blowing air into the fitting is what a professional spa service company will do.
Finally, visually look at the lower piping to imagine if all the water is drained. If you can see where the water would drain to, you should be good to go.