If you haven't linked your Twitter account to Medal, make sure to read this article to get you set up for Console Syncing!

Locate the Album on the Switch's Home Screen
Find a video file that you want to share and press A on your controller
Press A again to bring up the Editing and Posting window
From here, you can Trim your clip if you so desire. You can upload the full clip and still trim on Medal later! Next, select Post
Choose the Post to Twitter option
Most important: In the Tweet, make sure #NintendoSwitch is added (Added by default) otherwise Medal will not be able to detect the clip.
Once posted, the Switch will confirm that your clip has been posted to Twitter! After a few minutes, your clip will be uploaded to your Medal profile. You can now name and trim your clip directly in Medal!