Make a button at https://www.clickminded.com/button-generator/ (previously https://dabuttonfactory.com) and save it to your computer.
Click Upload in the Insert menu and select the button image file to insert it.
Click the Insert Link icon above the inserted image and enter the URL you want the button to go to.
The ability to add a button has been added to Sites.
Add a button
You can add a button on your site to take people to another URL or page.
On a computer, open a site in new Google Sites.
On the right, click Insert > Button.
Enter a name. This will appear on the button.
Paste a URL the button should point to, or choose a page on your site.
Buttons now have three styling options: filled, outlined, and text. By default, a newly added button will be filled, but you can can change the button’s style using the dropdown in its toolbar.