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.
Probably you seem to have a different definition of FOMO. Como is generally used in the sense that you know that the coin is over valued but you still end up buying it thinking you may not have the chance to buy.
It is often created by social media hype. Just have a look at the farming tokens and the people who fomoed into it.