It is very easy when the price is low, you have to buy, and when the price is high, you have to sell, nothing complicated.
It is not as easy as it looks, everyone wants to follow this technique of "buy low and sell high" but once you get into trading you will understand that you cannot predict the high and low of a coin. Also, coin will never fall before telling you or anyone. Hence, it becomes very difficult to trade with this rule rather you should fix your goals and sell as if you start following your greed to earn more from a coin you cannot earn profit from a coin.