Pretty simple to use, clamp the jig's fixed guide where you want the dado to be. Then insert the shelf piece and slide the adjustable guide up against the shelf. Lock the two knobs down. Then using a bearing guided router bit like this:

 

 

 

from Woodline USA. (Part #wl-1009D) run the router down one side of the guide arm and up the other guide arm. The dado should be a perfect fit to the piece. You should sand the piece thats going to fit in the dado before cutting the dados, otherwise you'll get a sloppy fit.

 

Making the Jig.

Starting with the raw supplies, I used 1/2" Baltic Birch plywood.

You'll need one piece for the two guide pieces and a piece for the rails. The piece for the guide arms should be about 5-6" longer than the widest shelf you want to dado. And 9-12" wide. The rails should be about 15-18" long and around 5" wide.

First thing is to cut the rails to width. First I jointed one edge (carbide jointer cutters come in handy for this). Then I ripped them to 2 1/4" wide. The exact width isn't important.

 

After the rails are cut its time to mark out the position they're going to be at on the guide arms.

Here's where I'm going to cut a dado in the guide arm piece for the rail to fit snugly in.

Getting this perfectly square across the guide piece is important.

Here the rails are fitted into their dado's.

 

 

Now to rip the guide arm piece into two pieces.

Now a slot needs to be cut down the middle of the rails for the hardware to ride in.

I cut the slots on the router table with a straight bit.

Then a counter slot for the head of the toilet bolt needs routered. Leaving the everything on the router table setup I swap to a 1/2" straight bit.
One final milling step, I drill a 3/4" hole in the rail where it's attached to the fixed guide arm. This gives the router bit somewhere to start.

Now time for some assembly. Attach the fixed guide arm to the rail.

Nothing fancy just some glue in the dado and a couple of screws to hold it while the glue dries. Make sure the two pieces are square to each other. Attach the other fixed guide arm with more glue and screws.

Then sand the rails where the adjustable arm is going to slide. Round over the corners slightly. Sand the sides of the rails until the adjustable guide arm just fits on easily. Apply some furniture wax to the dado on the adjustable guide arm so it'll slide easily on the rails. Finally mark for the holes for the bolt and drill a hole 3/4" of an inch from the outside edge of the adjustable guide arm.