Logging in
Go to the login form
Most venues will have a menu item near the top of the screen which says "Log in". Click that.

If you don't see it, look for a menu item which says "My account"; this will indicate that you're already logged in and can proceed to booking your session.
If there's no such menu entry, check also at the bottom of the page where some venues have their login link.
Enter your email and password
Now you'll enter your email address and password, then click "Log In" to log in.

Above: Login form
If that worked, great! Now you can continue to book your session.
Error messages and next steps.
If that didn't work, you might see an error message saying:
"Sorry, unrecognized username or password. Have you forgotten your password?"
This contains a link to let you reset your password if you don't know it. Check there isn't a typo in your email address first!
If you don't know your password, see reset your password.