Likewise in gambling, the summary of it is to trade only what you afford to lose.
My addition to it is to strengthen your emotion.
Actually your emotions play 95% on how you succeed or fail in trading. Emotions are two enge sword and it can safe you from hard loss and also it can makes you to close the winning trade too early. We can all become successful if all of us can predict the market accurately but our emotionals strength is what determines if we would succeed or not.
90% of trader failed because lack of knowledge about risk management, just like me when i enter to crypto trading i dont know how stop loss work in crypto exchange. Second thing we should be have knowledge about technical analysis, this is important to predict the trend of market.
Risk management is what traders should much focus on in other to be able to make profits from the market. I think we should not put more than we can afford to lose but many out of ignorance do that and that is why they keep losing.