Before draining your spa remove all filter cartridges and pour in a pint of HT Jet Line Cleaner for hot tubs. Use 2 pints for swim spas. Not required, but it works best when the water is warm.
Then turn on all the jet pumps and open all of the air valves for 20 minutes. Good news almost all spa pumps automatically turn off in 20 minutes.
Next simply let the water sit idle for about 45 minutes and then turn all the pumps back on for another 5 minutes.
Then turn off the power, drain the water and clean the shell. I like hosing down the shell walls and cleaning them with HT All Purpose Cleaner if necessary.
Then replace the filters, and follow instructions on “How to Properly Start Up a Hot Tub”