Open a professional's page, choose a service, pick a time and confirm. For most studios you can do this without an account — your name and email are enough.
What happens next depends on the studio. Some confirm automatically, so your time is held the moment you book. Others review requests first, and you will hear back from them.
You will get an email either way, and the appointment appears in your bookings if you have an account. If a studio has SMS reminders switched on and you have agreed to texts, you may get a reminder that way too.