Hi there, have you seen – https://support.redfactory.nl/documentation/restaurant-theme/integrating-cart66/
Once you’ve installed and activated the plugin, you’ll need to go into “Cart66 > Products” and start adding a couple of products and prices.
Once you’ve completed the steps above, jump back into the menu item you created previously (via the Posts tab) and go into a product as if you were going to edit it. Now in the top right hand corner of the editor page, you should see a dropdown option has appeared. Select the relevant product to connect the two, this will add the button on the main page.