Obviously, the basic folding pattern for a box-pleated ruffle is not too complicated. But... then there are the details. I have a few tips for the basic process and then what I figured out for how to deal with the end of the ruffle in three different situations (all seen on the 'Paris' apron).
Create and Attach the Basic Ruffle
First, you need to cut strips of fabric that will make the ruffle. How wide they should be depends on how deep you want the finished ruffle to be and what seam allowance you intend to use when attaching the ruffle. Add those two numbers together, then multiply the total by 2. That's how wide your strips should be. I cut mine 2.25" wide.
The total length of your strips needs to add up to a bit more than three times the length of the edge you will be applying your ruffle to.
Sew all the fabric strips together end-to-end, making sure to keep the seams all on the same side. Fold your new super-long strip over lengthwise and iron in a crease all the way down.
The material I used was 100% cotton, but from a business shirt that probably had a no-iron finish on it, which meant it was hard to iron a crease into it. Because of that and because I'm hoping this will keep the pleats nice and crisp for a while, I ironed some fusible hem tape (left over from IKEA curtains) inside the fold. If you want to use fusible tape, test first on a scrap of your fabric to make sure it won't show through. Also, iron the crease down the long center of your strip once before trying to put in the fusible. The crease will be a simple guide for you to lay the fusible neatly lined up.

1. Using some kind of guide to keep your pleats evenly sized (I counted pinstripes; slightly crazy-making), start to pleat your ruffle. Leave a bit of a tail at the end to make things easier. You won't need to iron every pleat at this stage if you've used fusible tape, just crimp with your fingers and put in a pin.
2. Stitch a line along the whole raw edge of the pleats to hold everything in place. Make sure your stitching line is inside of your final planned seam allowance so that it won't show later (e.g., if you've planned a 1/4" seam allowance, stitch less than 1/4" from the raw edge). Take your time with the machine because there's a lot of layers of fabric here. You might even want to use the handwheel instead of the foot pedal for some or all of this part (especially if a baby is sleeping in the next room). It's best to remove the pin from each pleat just before you sew over it, after it goes under the presser foot, so that it does its job keeping the pleat in place, but doesn't end up distorting the fabric permanently.
3. Now pin your ruffle to the edge of your fabric, matching up the raw edges and putting right sides together (your ruffle probably doesn't have a right side, but you know what I mean). Pin in the ditch of your pleats so you only have a few layers to go through. On top of the ruffle, pin a strip of single-fold bias tape, making sure the crease in the bias tape matches up with where you plan to put your final stitch line.
4. Stitch through all layers, right along the edge-crease of the bias tape. This is actually going to be easier than stitching across the pleats in the first step was, even though there are more layers of fabric, because the pleats are already secure and you can take your pins out before they go under the presser foot.
5. Iron the seam out flat so the ruffle comes off the edge of the fabric and the bias tape folds back over the raw edges. Pin the bias tape in place and stitch the edge of the bias tape to the main fabric.
Detail One: When the ruffle ends at a hemmed edge
Attach your ruffle to the proper edge, making sure to leave some extra ruffle past the edge of the fabric you're stitching to.
Use your seam ripper to free up the last pleat, both from itself and from the fabric, and make a small rolled hem (fold over about 1/8", twice) and stitch that in place. Realize that you will need this finished edge of the ruffle to line up with the final finished edge of the fabric, not the raw edge you see right now.
If you are finishing the perpendicular raw edge of the fabric with single-fold bias tape, leave about a half-inch of bias tape hanging off the edge. Stitch the bias tape onto the fabric. Your stitching line should end at the point that the fabric seam allowance meets the finished ruffle edge. Take a few backwards stitches to secure everything at that point.
Iron the bias tape back over the raw edges, tucking the tail of the bias tape back underneath the rest of the bias, so it's sandwiched inside. Stitch down.
Detail Two: When the ruffle is set into a square neckline
Align your ruffle so that the pleats appear evenly centered on the final opening (turns out, I had a little trouble with that, but that's ok). When pinning your bias tape in place, cut a piece of bias tape long enough to go around the whole neckline and start pinning by matching the center of the bias tape to the center of the ruffled edge. Don't pin the tails of the bias tape to the rest of the neckline yet. Stitch all three layers together along the ruffled edge, starting from before the cut opening (the seam allowance amount before) and ending the same amount past the other end. Backstitch at both ends for security.

(click on this image to see it larger)
Now, what we need to do is make the bias tape turn a corner that won't cause a pucker, while also enclosing the raw edges of the ruffle and the neck opening.
1. Clip into the corner of the cut opening (to the end of the stitching line you just made when you attached the ruffle).
2. Holding the neckline right side up in front of you, flip the ruffle up into place.
3. Tuck the bias tape and end of the ruffle behind the raw edge of the neck opening.
4. Fold the whole side of the garment over onto the neckline, which will expose the seam you will be sewing. The ruffle probably has extra seam allowance, so don't try to match up the raw edges of the neckline and the end of the ruffle, just make sure they are parallel.
5. Holding things in place and flipping the whole sandwich over, fold the bias tape down, away from the neckline at a 90-degree angle from the edge which has the ruffle already attached. Then, fold the bias tape back up, so that it will now be lying in place along the raw edge of the side of the neckline. But, leave a little pleat in the bias tape when you make this last fold up. This little pleat is what will allow your finished neckline to lie flat, without puckers at the corners. (I cheated one side and did the other just right, I'll show the comparison at the end.)
6. Put in a pin to hold it all together. If you put the pin in aligned with the seam you'll be stitching (instead of perpendicular, like I did), you'll be able to try folding the bias tape back into final position to check that you've left enough of a little pleat in the corner. (The photo in step 9 shows the effect of a little too small of a pleat. Oops.)
7. The view from the other side. Note that the garment is still folded to allow the seam allowance of the neckline to line up with the ruffle's raw end. Keep it that way while you sew.
8. Stitch the bias tape to this half of the neckline, starting a little ahead of the ruffle's attachment stitch line and backstitching up to meet it exactly in a corner. The tail of the ruffle will be caught between the bias tape and the garment fabric.
Do all the above on the other side.
9. Fold the bias tape back into place and iron it down. Trim off the excess tails of the ruffle and stitch the bias tape down, which will neatly cover all raw edges.
Hopefully, both your corners turned out nicely. :)
Detail Three: When the ruffle is at the top of a patch pocket
This one is pretty simple! Attach the ruffle to the top of the pocket the same way as ever, making sure to leave a good tail of extra ruffle hanging off at each end. Iron all the edges of the pocket under, including the edges of the ruffle (you may need to use the seam ripper to free the last pleat so you'll be able to fold the edge under). Stitch the pocket to the garment on the machine, leaving the ruffle free. I found it easiest to attach the edges of the ruffle by hand. Fold them under twice and stitch to the garment fabric, leaving the front layer of the pleats free.