Because we now have a strip of our Tartan that represents the pleats from apron to apron we could, if we want, start pleating like a traditional kilt maker does at the left edge of the outer apron. Or we can start at the back center pleat and work both ways.

If you started with double-width fabric you can now join your pieces and layout your pleats as if you had a single piece of fabric.
Here is a layout starting with the center back pleat.




If you chose to pleat your kilt like a traditional, from the apron, here is what the first three pleats would look like.



And showing how you would move the layout strip over and lay out the fourth pleat.



And the fifth



And the sixth etc.



Just move that layout strip along your fabric until the Tartan pattern lines up and mark the next pleat.