That's an easy one Adam. You take the shape you want and cut a hole in a piece of MDF to match the shape but offset by the correct amount to allow for the guide bush.
Let's work with a few numbers. Outside diameter of your guide bush = 20mm. The dia of the cutter is 12 mm. The sum is, 20 - 12 = 8. Divide 8 by 2 = 4. This 4 mm is the offset required.
To cut out a square recess (e.g., a mortice) in a piece of wood 60 mm long by 20 mm wide by 10 mm deep you cut a rectangular hole in a piece of suitable MDF 68mm long X 28mm wide.
Put this over the spot where you want the recess and clamp it in place. Put the guide bush and cutter on and in the router. Drop the router on your pattern with the guide bush in the cut hole. With the router switched off push the cutter down until it rests on the wood you want to cut. Call this zero. Set the depth of cut required, i.e., 10 mm.
Raise the cutter off the wood. Switch on the router and push the guide bush tight against the side of the slot in the MDF. Plunge in. Move the router clockwise around the MDF pattern. You're done with a round cornered mortise 60 X 20 X 10 mm. Move on to the next one.
In your case you want to drop an elliptical decal into a shallow recess. The principal of making a pattern is the same though. I'd use something like a 30 mm outside diameter guide bush and a 6 mm cutter which would give you an offset of 12 mm. You could use an 18 mm guide bush and a 9 mm cutter giving an offset of 4.5 mm.
You simply need a means of drawing the elliptical decal shape onto your intended pattern. Often a washer of the right diameter can be used to trace the offset. Place the washer next to the decal, a pencil in the hole in the washer and roll the washer around using the pencil thus transferring the oversize pattern as needed. Cut this out and off you go. Slainte.
You may have to put your tag on the work before it is made in some cases as the jig may not fit inside some places . By the way where did you get the tags from ?
