Once you're reserved for class, signing in to class is easy.
On your app, if the gym allows self check-in, you can swipe left on the class you reserved and it will pull up a 'Check-In' or 'Cancel' option. Click 'Check-in'.
The gym may offer a front desk kiosk that shows everyone that reserved for class and you can check-in right from the kiosk.
If the gym does not offer self check-in, a coach will check you into class.