There are two methods for two different types of investors. What it means for short-term and long-term traders If you are a short-term trader, then for you it is a little different but easy to follow. That is, buy the deep, hold the deep for a period of time, and then sell at the green when you believe the markets are stable. the simple theory for short-term traders, which I think every expert trader is instructed to follow. 
 
Now comes to long term traders. For them, they don't need to follow any instructions. They just have to enter at any time of the market and wait for their golden moment to take their profit. But in this case, heres my personal small suggestion: whenever you enter the market, please remember that time shouldnt be a bullish time in the market. If that time is a bearish and stable one, that's a good time because you can see your portfolio green soon. But if you want, you can enter at a bullish time too. It's your choice. I will just follow this method. Thank you.