You can green an entire round-trip flight, including all legs, via a message on Slack. Just start a command with "/green" and include the airport codes for all airports along the way. For example, this is how you would green a round-trip flight between San Francisco and Frankfurt, Germany, with a connection each way in Atlanta:
/green my flight from sfo to atl to fra to atl to sfo
For shorter (or longer) flights, just include whatever airport codes you need. For details of how it works, see the article "How does Cloverly's API calculate the carbon impact of air travel?"