Go to Settings, then click ServiceM8 Add-ons
Go to Online Booking, and click the switch to turn it on
Add the HTML snippet to your website
Next to each button is a snippet of HTML code which will display the button. If you have access to edit your website directly, you can copy/paste this code into your website to add the button. Otherwise, you can send the HTML code to your web designer who should be able to add the button to your website for you.
Alternatively, if you don't want the hassle of manually editing HTML, you can add a Book Now button to your business's Facebook page.