Step 3: Mark the dovetail groove
Decide how wide you want the dovetail to be. It should be narrower than the thickness of your stock, of course, especially the guide stock if you're using different lumber for different parts. Set your combination square to a distance that is just a little less than halfway across the base, place the square along one of the long edges, and make a mark. Place the square on the other edge, and make a mark. put your guide piece along the marks to see that the marks define a space narrower than the thickness of your guides. If it is, you're ready to rock. Otherwise, adjust the square and try again.
After playing with your square for a while, take it and run it along one of the long edges while holding a pencil at the top of the square. This will make a line you will use to cut the groove. Do the same thing with the other edge.
You now have two parallel lines (assuming the edges of the board are parallel) that define a centered groove. Nice job! Now extend those lines down the side of the board, set the combination square to the desired depth, and mark that on both the end sides as well.