My father is 88 years old and has studied U.S. stocks for nearly 33 years.

He summed up some simple rules for beginners:
Price drops 5%→ Hold
Price drops 15%→ Buy 10%
Price drops 25%→ Buy 25%
Price rises 5%→ Keep holding
Price rises 15%→ Keep holding
Price rises 25%→ Sell 10%
Price rises 35%→ Sell 20%
Price rises 45%→ Sell 30%
Price rises 60%→ Sell 40%
Price rises 100%→ Sell everything
Discipline + patience = steady long-term growth.