There are many QR code generators available now; I’m using Google’s generator because it gives a convenient option to shorten the URL as part of the process. (A long URL creates a “busy” QR code, which is more difficult to scan than one less cluttered.)
Example of An Impossibly Long URL (from the Library of Congress)
1. Copy the URL of the webpage.
2. Go to http://goo.gl. (You must have a Google account.)
3. Paste the long URL into the box.
4. Click Shorten URL.
5. Click Details.
6. Copy (or save) the QR code
Click here for complete instructions with additional images. (PDF file)