Let's assume you know reasonably well when a market crash will occur. There is significant risk associated with trying to 'time' the market, and you should make yourself familiar with those concepts before proceeding.
If you are an average 'private person' investor, there are a few ways to make money from a crash, and here are 3 significant ones:
- You could short a particular stock or diversified fund that you expect to drop.
'Shorting' means you borrow a share from someone else in the open market, with the promise to return that share back to them. Immediately upon borrowing the share, you sell it to a 3rd person in the open market. So you borrow someone's share of Tesla, currently trading at $390, and sell it for $390. Let's say next week Tesla announces some big problem, or the economy goes downward and all car makers look like they will not be able to sell as many cars, or whatever. Tesla shares then trade at $360. You buy a share of Tesla for $360, and give it back to the person you borrowed it from. Now you have $30 cash in your pocket.
[note - shorting is higher risk than regular stock ownership, because (a) your losses are unlimited, for example if Tesla invented a Vibranium car, and stock rose to $800, you would lose $410 for each share you shorted, which is larger than the amount you originally had; and (b) markets generally go up in value, so shorting is a specific moment-in-time assessment, and shouldn't be done on a consistent long-term basis, unlike simple diversified investment in the stock market]
You could buy a 'put' option. An 'Option' is the right to buy (a 'call option') or sell (a 'put option') something at a specific price, at some point in a specified period of time. For example, buying a put option for AAPL with a $120 strike price that expires in 6 months, would give you the right to sell a share of Apple stock for $120, at any point in the next 6 months. Right now, with Apple trading at $125, you wouldn't want to exercise that option, but if the price drops to $115 in December, you could choose to exercise the option, and sell a share of Apple for more than you would be able to in the open market [really when you exercise the option you would just receive $5, you wouldn't need to actually transfer shares]. The benefit of an option vs shorting the stock, would be that your risk is well defined - you pay some money up front for the right to choose whether to exercise the option, and if the anticipated crash never happens, you don't lose anything further.
You could buy something you think will do well during a bad economy. This could mean gold [common 'market wisdom', but see Bob Baerker's excellent analysis of why this is probably not a great idea], or maybe 'vice stocks', like liquor and gaming which often perform well when people want to drink away their sorrows, or whatever else you think is associated with the crash incoming [maybe if you saw a Covid crash, you could have thought hospitals would do quite well? etc.]. The problem is that you may not know how these things will perform even if you do accurately time a market crash. So there is additional risk here.
You could buy stocks right after the crash occurs. If you have, say, 10k invested in the market today, you could sell it, or otherwise if you just have 10k lying around, you could just hang onto it and wait. Then 'when the crash happens' [this doesn't just mean 1 day of bad results, and can mean many different things that are very very hard to predict, including maybe a 6 month protracted drop in value], you buy things that you think will 'rebound', probably because you believe that panic during the market crash will make people emotional and overreact, thus undervaluing shares they used to own. Note that this practice often has another name 'catching a falling knife', meaning you could easily buy something on the way down, it continues to drop, and never fully recovers.
If you wanted to really 'make a big impact' using any of the above, you could go full-on risk heavy, and trade on a margin account / borrow funds, and invest more than you have in available cash. Of course if you are wrong, and you took out a 2nd mortgage on your house and have losses preventing you from paying it back... the results can be disastrous.