Select the Design tab at the top of your Websites app.
In the preview window, go to the page with the text you want to turn into a button.
Highlight the text and select the Create/Remove Button icon from the pop-up box.

Paste a URL or choose a page from the drop-down menu.

Optional: Click the "Open in a new window" icon to have the button open in a new tab.

Select the blue Save button.
To adjust the button's appearance, click on the Buttons option in the right-hand sidebar and expand the section. Here you can edit the design settings for the buttons in that content block. Click Save when you're finished.
