The general rule used to be"buy low, sell high".
It is quite simple and can be done by anyone, but in its application it is very difficult.
It is also about how to trade knowledge, master TA and FA and do risk and money management.
Of course, it is not easy, the OP only asks the right time to sell and buy, but it depends on where the target is to sell and sell.
Psychology will also really play an important role in making decisions to buy and sell,
this is not easy because many already know enough about the basic science of trading but are still stuck in bad trading.
So, the point is to learn how Trading works by understanding TA and FA Analysis,
knowing how crypto market trends work and lastly more trading experience.