With all of the grooves cut in one stile, you'll need to build a second jig (or disassemble the first jig) and repeat the same instructions from the past two steps in the opposite 45-degree angle.
Once you have a matched set of grooved stiles, dry-fit the slats in one of the stiles as shown in the picture on this page.
Then, evenly brush a bit of glue into the two mortises (three if you're making a door) on this stile and on the appropriate rail tenons, then slip the tenons into the mortises. Immediately wipe off any glue that squeezes out of the joints.
Next, brush some glue into the mortises on the remaining stile and on the two (or three) remaining tenons. Position the tenons into the mortises and, working quickly from one rail toward the other, place the individual slats into the proper grooves of this stile.