Uploading from a desktop is the main practical reason to do video work in Basecamp rather than on your phone. The files are already there, and you are not waiting on a mobile connection.
Uploading
Select a file, give the video a title and write a description, then upload. Progress is shown as it goes, and both a file and a title are required before it will start.
Longer videos take longer to upload and process, and Basecamp says so on the upload screen. Processing continues after the progress bar finishes. A video is not ready the instant the upload completes.
Narrate it if you can
Basecamp gives this advice on the upload screen: if you talk over the video, say what you are doing as you demonstrate it. Speech is captioned automatically, so narration that describes the movement works for clients who cannot see the screen.
There is a real difference between saying "like this" and saying "hinge at the hips, keep the bar close to your legs, stop when you feel the hamstrings." The first carries nothing for anybody who is not watching. The second becomes a caption track that works on its own.
It costs nothing at filming time and it is the difference between a clip that works for everyone and one that works for people who can see it.
Format and size
The same guidance applies as on the phone: MP4 with H.264 video and AAC audio, 1080p, a constant frame rate and standard dynamic range rather than HDR. Basecamp rejects files over the maximum before upload and tells you the limit.
If a video uploads and then disappears, it usually failed processing. The most common cause is an HDR recording. Re-export as SDR and try again.
The free limit, and where to fix it
Accounts on the free plan have a video upload limit. Reach it and Basecamp tells you where to go next: upgrade your Workout Titan plan in the iOS or Android app to upload more.
You cannot upgrade from Basecamp. Subscriptions are bought through the app stores, so this is one of the few jobs that has to happen on your phone. Worth knowing before you sit down to build a library and hit the limit halfway through.
Name it so you can find it
You attach videos to exercises by searching your library by name. A library full of camera filenames is hard to search, and renaming 20 clips later takes longer than naming each one as you go.