Position this board with the center mark directly under your router bit on the jig. Remove the router, and make a pencil mark on the jig that that corresponds with the first 90-degree mark you made on your test board. This will be the alignment mark for positioning your stile when you cut the grooves (as shown in the image above).
Put the router back on the jig and move it forward to the point where the edge of the bit meets the far 3/4" mark on the diagonal. Find a short piece of scrap 1x2 and position it against the router base on the jig and screw it into place. This is a stop to keep you from routing too close to the edge of the stile.
Move the router toward you in the jig until you reach the other 3/4" mark on the diagonal line and attach another stop block against the router base for the bottom side of the jig. You should now have 1-1/2" of travel perfectly centered on the test stile.
Set your router to make a 1/4" deep cut and clamp
the jig and test stile to your table. Route a test groove, remove the stile and dry-fit test the fit with one slat. Adjust the jig if you aren't happy with the fit.
Next, we'll set up one stile for cutting by making a perpendicular mark every one inch along the edge of the stile to be grooved. Start by making a mark at the half-way point of the length of the stile. If you have an odd number of grooves to cut, make one mark on each side of the center every inch until you have the requisite number. If you need to cut an even number of grooves, make a mark 1/2" on each side of the center line, then a mark every inch on each side until you reach the desired number of slats.
With all of the marks completed, clamp the stile to the table and route the first groove. Then, remove the clamp, slide the stile to the next mark, clamp and route. Continue until all grooves have been routed.