@startuml
skinparam handwritten true
You -> slack.com: 1. I would like to access my files on Google Drive via your interface.
slack.com -> You: 2. You should apply the "Authorization Code" from Google for me first.
You -> account.google.com: 3. I would like to permit slack.com to access my files.
account.google.com -> You: 4. Are you sure?
You -> account.google.com: 5. [Y]
account.google.com -> You: 6. Okay. Here is the "Authorization Code." Plz give it back to slack.com now.
You -> slack.com: 7. You can do what I asked now. (With the Authorization Code which is just received from Google) 
slack.com -> account.google.com: 8. I would like to exchange the "Authorization Code" for the "Access Token."
account.google.com -> slack.com: 9. Your Access Token here. 
slack.com -> drive.google.com: 10. I would like to read files which belong to one of my customers. Here is the Access Token.
drive.google.com -> slack.com: 11. Hmmm...the Access Token is vaild. Here is list of files you asked.
slack.com -> You: 12. Grant succeed. You can see your files stored in Google Drive from my interface now!
@enduml