If you have an Excel/spreadsheet file with your clients' names, numbers and email addresses, import them into Bookem. The benefit of doing this is that booking an existing client in is just a matter of quickly searching for and choosing the client your booking in.
Synchronise your Google (or other external) calendar with Bookem to block out the slots that are already taken by appointments (there should be sufficient text in there that's imported into Bookem as to who the appointment is with and what for). More on connecting your Google calendar.
Now that you can see on your Bookem calendar which slots are booked out and with whom, it’s a simple matter of properly booking the clients on Bookem by booking a slot at the same time, i.e. concurrently, by clicking on the open space next to each blocked out slot.
You know you’re done when you can see each blocked out slot from your external calendar has a matching concurrent booking in your Bookem calendar.