One of the most important things in this process is to make sure your fabric is torn on the straight grain. If it's not, it will be a disaster!
Roll your fabric on your dowel as perfectly straight as you can. If your fabric isn't perfectly straight, your pleating won't be perfectly straight.
It's very important to make sure fabric gets started perfectly even vertically and horizontally. Line up torn edge with a groove in the roller - for me, that is one groove past my last threaded needle. Make sure your fabric edge follows that groove the entire width of the fabric, on the vertical as you pleat. It's equally important to watch your fabric the entire length, making sure it goes thru the pleater evenly on the horizontal.
Engaging your left hand is very important as you pull that fabric thru, keeping it taut as you pleat and making sure the horizontals of the gingham are not getting off grain as it pleats. In the insert photo your needles should be all following the same "row" of the gingham the whole width of the fabric.
If you follow my steps, then your pleated gingham should turn out like mine! All your pleating guide threads should follow the same line of the gingham the entire width. You will also have very little moire effect if you keep everything perfectly straight as you pleat.
I hope the next time you pleat gingham you can do it with success and that my tutorial has been a helpful tool! :)