You can't do this in the free version of TurboTax Online, so you'll need to upgrade.
Once that's done, to enter your DoorDash expenses, sign in to TurboTax and
- Select Pick up where you left off
- Using the searchbox in the upper right, search for self-employed expenses and then click the Jump to self-employed expenses link
- When your DoorDash income appear on the Your 2020 self-employed work summary page, click the Edit button
- You'll see an Add expenses for this work box below. Press that and you'll be able to add your expenses.
If you have some expenses that don’t quite fit into the expense categories that are listed, it's okay to enter Miscellaneous Expenses.