This is just a simple dovetail marking guide. I've been using one just like this for the last 10 years and decided to model up a printable version to share.

There are two options to choose from. An 8* angle suitable for pretty much everything, and a 10* angle if you want a little more dramatic looking tail. Personally, I use 10*, but I know some woodworkers prefer the shallow angle.

Using it is simple. One side is angled for marking the face and the other is straight for the end. use a sharp pencil and follow the lines as you saw.