You can see whether you need to make a booking and how to do so for a particular course or free training by checking the partner profile on the website or in the app under "important information".
You make a booking in the app directly, for example for all live courses, if you see the button "instant booking".
In some cases, you need to contact the partner via phone or email in order to make a booking. If you make a booking and cannot attend be sure to cancel the booking as soon as you can so other members can take your spot. If there is no information on how to make a booking then it is not necessary for this particular activity.