The guide attaches to the end of your file with a setscrew. You set the fleam on the top scale, the rake on the bottom scale, tighten the thumbscrews and, presto, you have an excellent visual reference for both angles. There is a comfortable place on the end to hold the guide with a thumb and forefinger.
Many experts don't use a guide at all but I am not able to maintain accurate rake and fleam angles without a visual reference. There are a number of shop-made solutions that are perfectly adequate, but I strongly prefer this guide for its simplicity, accuracy and ease of use. It is very well made, typical of Lee Valley, and I particularly like the fact that it is based on a historical tool in the company's collection.
If you are a beginning or intermediate saw filer, I strongly recommend that you purchase this guide when it becomes available, which should be very soon. I really like it.
On a related matter, quality saw files have become hard to find. My local hardware store carries some, but they are junk. The two sources for good files that I know of are Lee Valley and Lie Nielsen.
The only other thing that you might also consider buying is a saw vise, either vintage or this new one, which I've read is excellent. For reasons I will explain, I think a homemade one is better and it is certainly a lot cheaper. My version will be the subject of the next post.