How do I do Delta online check-in step by step?

You can do it on both:

1. Go to the Delta website or open the Fly Delta app.
2. Choose Check In and enter your confirmation number (or SkyMiles number) and name.
3. Confirm passenger details and any required travel documents.
4. Select seats or upgrades if offered.
5. Add bags and pay any fees.
6. Save your mobile boarding pass to your phone wallet or print it.

Doing it online saves time at the airport, especially if you only have a carry-on. More on Delta online check-in is explained here.