Step 1: Identify and mark the center front of your pattern. (I would trace over my pattern to create a new one, making sure to replicate all the notches etc. Don't cut it out or anything yet and leave plenty of room next to the center front to add the pleat allowances.) (We are pretending that my center front lines etc are all straight in these drawings, btw.)
Step 2: Add your pleat allowances. I wanted my pleat depth to be 3 inches so I added 3 inches to the fold line and then another three inches to create a second center front. (You will create an inner and outer center front for this. It will make more sense later.) You should mark all of your fold lines and center fronts. This will also be cut on the fold, so mark that too.
Here is what my actual pattern piece looks like:
Step 4: Bring the outer center fronts to meet at the inner center front marking. This is where you create your pleat.
Here is how that looks from the front. (The top part of my pleat is sewn together.)
Here's how it looks from the back:
Step 5: Open or closed. At this point you can decide if you want to leave the pleat open, or if you want to sew part of it closed. (I did the latter on my skirt.) If you want to leave it open, just fold it, press it down at the top, and baste along the upper edge to keep it in place until you add your waist treatment. If you are going to sew part of the pleat shut, just mark the amount you want to sew along the outer center front lines, sew down that far and then match that center seam along the inner center front of the pleat and baste it at the waist.
I like a crisp pleat, but I would not advise ironing down to the hem until you have finished it. (Your hem finish should not be too bulky either. You want to be sure that you can get a good fold on the pleats.)
There are ways to get more complicated with this, such as using inverted pleats as kick pleats at the back of your skirt, but this is a very simple application.