Please explain to me how , in the short term, one

Please explain to me how , in the short term, one can easily sell high ,buy lower , without relying on luck ? Specifically,why would you be sure of the price movements of CYDY on any particular day?
I retired very early,and have lived solely off stock market earnings since(20+years) ,never would I claim buy low sell high is "easy peasy" . But if you know better I'm happy to be educated.