When an email is open
Click on the green Snooze Email button on the top menu
Another menu will appear allowing you to set a time for when you want to deal with the email. There are five pre-set scheduling options, but you can also pick the exact time you want (1). You can add a note to yourself about the email you are snoozing (2).
When an email is not open
Click on the little clock icon that you see beside each one of your emails. The clock will open the Snooze