The key to making louvers is in the grooves for the slats in the stiles. If the grooves aren't properly spaced or parallel, the louvers will look odd.
There is a simple solution to this potential problem: a louver jig for your plunge router. that can be built out of scrap stock, and will ensure every groove will be perfect.
If you check back to the plans you downloaded in step 1, you'll find a four-view measured drawing of the router jig.
To begin, you'll need two long (at least 24") boards that are a bit thicker than the width of the stile that you'll be routing. These don't have to be solid boards; you could use a couple of pieces of scrap 3/4" plywood and shim some other scrap stock beneath the two boards to raise them to the proper height. Just make sure that the channel between the two long boards is clear.
Next, position one piece of 1x2 in a 45-degree angle (using a layout square) a few inches to the right of center on the two parallel boards. Adjust the width of the parallel boards so that they match the thickness of the stile (on edge - this would be 1-1/2" if you used 2x4s to make the stiles). Affix this 1x2 to the two long boards with screws.
Now, position your router with a 1/4" radius straight-cutting bit on the two long boards, butted against the left side of the angled board you just attached. Position a second board parallel to the first angled board, on the other side of the router base. Attach with screws.
Next, you'll need a piece of scrap the same size as the stile to be grooved. Mark a line across the width of the same edge of the board as the groove to be cut in the stile. Make a mark at the center of this line, then make a 45-degree mark (in the same direction that you positioned the router guides in the last step) through the intersection. Make a mark at 3/4" in each direction from center point on this line.