Alan, I have never heard of the Carey 300-2 hinge template, but, this is template routing plain and simple. You will fasten the template to the door, perhaps with clamps or double sided carpet tape? The cut out section is designed to work with 2 hinge sizes, and here is how you figure out what they are: Starting from the center point of the router a 1/4" bit will make a cut 1/8" on each side of center, a 1/2" bit would make a 1/4" cut on each side of center. With me so far? A 3/4" bearing will ride 3/8" from center, the 5/8" bearing would ride 5/16" from center. Since we know these two dimensions and you have a template of a set size it will be easy to figure out what size hinge fits the mortise cut. Since your bearing will ride 3/8" from center, and only 1/8" of that will be cut, we get a difference of 2/8" or 1/4". Remember this 1/4" will happen on both sides of the center point, so the length of the hinge will be the size of your opening minus 1/2". Using the 5/8" bearing will give different results. This time your bearing will ride 5/16" from center and 1/8" of that will be cut. This means you will have 3/16" difference on each side of center, or an opening 3/8" longer than the hinge length. Knowing this will allow you to build your own templates for other hinges, or figure out the distance a rabbet will be cut into the edge of a board.
Routing the mortise is straight forward. You set the bit depth to match the thickness of the hinge plate, and follow the opening in the jig in a counter clockwise direction. Once you have made the outside cut you slide back and forth to remove the rest of the wood. If you have round cornered hinges you are done. If you have square cornered hinges you will need to take a chisel and remove the wood in the corners. There is also an inexpensive tool called a corner chisel which will mark a perfect corner with one tap. All this sounds a lot worse than it is, you should have no problems. Just by way of reference Porter Cable also makes a hinge template.
"Living in the D" (this means Detroit!)
"It's fine to disagree with other members as long as you respect their opinions"