Once the extension is installed, you can schedule a message in any Slack channel using the /schedule command.
The command takes two options - the message you would like to send, and the time you would like to schedule it.
For example, if I wanted to send the message "hello" in 2 hours time, I would post:
/schedule hello in 2 hours
If it is your first time using the command, you will be asked to authorise the extension to schedule messages on your behalf.