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.
Well it depends on the coins that you're going to invest in.
If you chose new coins then its better to buy once the price is going up, its a good timing to join the hype, however new coins are much risky compared to old coins and majority of them are just a pump and dump scheme.
On the other side when it comes to well-established coins buying because of FOMO is not a wise idea. If you have knowledge about the coin and its past history then you'll understand how it works.