If you only run with Stryd, Stryd starts recording as soon as you run. You can stop the run by taking Stryd off your shoe and setting it on the table. Stryd will shut off automatically. Sync out your data with the Stryd Mobile app after your run.
If you use a watch or phone, look for the power number on your device! You know you are recording if you start an activity and can see your power.
If you have any problems, scroll up to the 'Run with Stryd' section and look for instructions for your device.