How do I record an activity on Garmin Connect?

Which Garmin device do you have? Without knowing which device you have, these are the general steps:

• Install Garmin Connect Mobile app on your phone
• Add your Garmin device to Garmin Connect app
• Choose an activity on your watch, press start. When finished, press stop, then save.
• Your activity will get uploaded to Garmin Connect
• If no mobile device, you can sync with Garmin Express using the supplied USB cable and a personal computer
• You can also look up the appropriate user manual on the Garmin Support site.

I suspect he wants to record the activity on the mobile app directly, without any garmin device. In which case it is not possible.