Using Apple Workout, Strava or another app as well
Keep recording in Apple Workout, Strava or whichever app you use, and check in on your phone. Regulars reads the workout from Apple Health or Health Connect afterwards and matches it to your session.
A watch runs one workout at a time, so if you record in another app, don't also tap Start tracking in Regulars.
Workouts with no check-in
On You, open Activity history and tap Show workouts from Apple Health (or Health Connect). Workouts from the last 7 days with nothing in Regulars yet are listed. Tap Add on one and it counts like a check-in. Only you see this, and it's read on your phone only.
Still stuck? Tell us what happened, or email [email protected]. Say whether it's on iPhone or Android, and which version of Regulars you have (it's at the bottom of Settings in the app).