Winterizing Pool Chemistry: A Complete Checklist
In order: clean thoroughly first, balance pH (7.2-7.8), balance alkalinity (80-120 ppm), check/adjust calcium hardness, shock the pool and give it 24-48 hours to work, add a winter-specific long-lasting algaecide, add a stain/scale preventer if you have hard water, lower water level and winterize equipment per your climate, then cover securely.
The biggest mistake: closing with chemistry already out of balance. Whatever state the water's in when covered is roughly the state it stays in — minus the chlorine, which depletes over winter even with algaecide.
Pool Balance's free calculator lets you run pH, alkalinity, calcium, shock, and algaecide doses for your exact pool size before closing up, instead of estimating five things at once.