My "catch the bottom" principle is like this:

Buy separately: don't buy all at once, buy in several times, which can reduce the risk of the market continuing to decline.
Observe the overall situation: it is best to wait until the market sentiment stabilizes or when the major stock indices start to rebound before considering entering the market.
Set stop-loss point: If the loss reaches a certain level, stop the loss in time to avoid further losses.
Look at the fundamentals: Choose good companies that have been oversold due to market sentiment, but still have good fundamentals.