I was only able to accomplish this from the app. "Check-in instructions" is located under Guest Resources. Options are different on the app than on the PC. When I went to this section on the computer, it didn't give me this option. I was only able to select that I have a self check-in keypad. The app allowed me to add photos and instructions.
Airbnb has my street name spelled incorrectly in their map system. Guests can still find the home, but I added a photo of the street sign to show the correct spelling and that I'm not providing incorrect information. I also added a photo of the keypad & front door so that they know what to look for with instructions on how to use it.
Go to your listing (hosting/editing side) within the app and scroll down to Guest Resources