You pay a small fee to an online prop firm, and in return, they set you up with an evaluation  demo trading account that comes with a amount of trading capital. For example, you might get $50K to start with. 

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During your evaluation stage, you will be given a specific target. Let's say the target is to reach $53K. If you manage to hit that target, congratulations! You become what's called a "funded" trader and the prop firm will now allocate you $50K of real money to trade with.

When you make profits from your trading, a good portion of those profits goes directly into your pocket while the rest is kept by the firm that provided the funding. Normally, these profits are split with 80% going to you, and 20% to the prop firm.  (Profit splits vary for each firm).

What's really great about prop trading is that you don't have to worry about risking your own capital. If you happen to incur losses, it's not your responsibility. The prop firm takes on that risk. Your personal finances remain protected.
The only costs you will incur throughout this process comes in the form of initial sign-up fees, which can be as little as $40. If you encounter any setbacks or fail a challenge, you will have the option to reset and begin again, either free of charge or for an additional fee.