This process only works if you are canceling your session 24 hours before its due time.
If you are trying to cancel inside of the 24 hour period then you will be charged a cancellation fee which is half of the session price.

Once you book you should receive a conformation email to the email you used when booking.
This is how it should look.
If you DID NOT get a conformation email then you more than likely mis typed your email.

After locating your email, then click the button that says "click here"

After clicking the "click here" button you should get this message.
Now hit the "Cancel" button

Now click yes

So now your session is cancelled. Go back to your email to confirm.

This is the email you should receive.

If you have booked multiple sessions then this must be done for each individual session.