The scheduling process begins once you have completed composing the email. In Gmail web UI, you click on the down-arrow next to the blue Send button and then, click on the Schedule send option that appears above.
In the Gmail Android app, you would need to click on the 3-dot (more) menu in the top right and from the options, select Schedule send.
Here are the screenshots from the Gmail web UI:
The Gmail Android app:
and finally, the Gmail iOS app where the Cancel option is the one to the left of the Trash can icon above the message:
In conclusion, a few words about a little icon introduced to Gmail very recently. A support icon, that looks like what's highlighted in the screenshot below: