The simple rule is "buy at the bottom and sell at the top". But i don't think this applies to all coins, it only applies to solid coins like bitcoin, ethereum and the likes, because they have stood the test of time and would surely find their way back up. But most of these new coins, if you buy at the bottom, the price might still go lower and keep you trapped there. so it is better to wait and see a clear move up before you get on the train to avoid fake outs.
Trading little-known coins is very difficult. Because their price is most susceptible to manipulation by big-money players. They can throw a few million dollars into creating a hype around one of the weak coins, they will start buying it, the price will rise quickly, which will arouse the interest of other people. When people start buying this coin EN masse, the big player will sell them all their coins and leave. Because of this, the price will fall forever and will no longer grow. Such situations are quite common.