QRCode Monkey is a good tool and I don't have any problem recommending it. But if you're looking for an even easier way to make a QR code for a Google Form, there is a relatively new option for making QR codes built right into Google Chrome.
When you're viewing the URL for a Google Form or any other web page in Google Chrome you'll see a small QR code icon in the right edge of the address bar. Simply click on that icon and a QR code will be generated for that page. You can download the QR code as PNG image file to download and print or download and insert into a document. This is probably the quickest and easiest way to create QR codes for Google Forms.
In this short video I demonstrate how to quickly create a QR code for a Google Form without having to use any third-party tools.