Adding your client portal to your website via either a book now button or by directly embedding it into your website is a great way to promote client bookings and save you administration time.
A book now button is the most straight-forward method for adding the portal to your website. You should be able to easily do this depending on how your website is configured and through which service. At it's most basic, you're simply adding a link to a button that leads to your client portal URL.
You can get your client portal URL from Setup > Client Portal.
Embedding your portal is slightly more difficult, but managable. In order to do this you simply need to insert the client portal using an iframe. An iframe is a HTML tag that allows you to embed external websites into the webpage.
At it's most basic, the iframe should look something like this:
<iframe src="https://my.yourportalurl.com" height="75%" width="75%"></iframe>