It's pretty simple. You need a startup of at least $20 (don't worry, you'll make it back later if you're trying to make money.)
1.) Make an order that's under $20, but enough to use up as much of the $20 as possible. 2.) When your food comes, eat that shit fast. 3.) Go to your order on the app, click "help" in the top-right 4.) Select "Issues with Order", then "Missing Items" 5.) Select everything on your order as missing (WHOLE ITEM) 6.) DoorDash will automatically give you back credits, which you can sell use.
Now, if you're wanting to make money, tell your friends that your parents send you X amount of DoorDash to "make sure you're eating", and then give them like a 40% discount. (Have them Venmo/Cashapp you money then use the credits for their order.) Or if you're wanting to save money/eat cheaper, just use the credits yourself and repeat.
If you're doing it for the money, make a new account, and re-invest some of the cash earned from your friends, and repeat the process.