Your measurements will depend on the effect you're going for. I wanted vertical ruffles, and I wanted them about 5" wide, so that was where I started.
For vertical ruffles, measure the length of your skirt from waist to floor. If you want them horizontal or diagonal, measure that distance. Write this number down.
Your fiber optic fibers should be about 2-3x this length. For my skirt, I used about 7 feet of fiber.
When you order the fibers, be sure to order an extra foot or two in length over what you think you'll need -- you can always trim the fibers down but you can't make them any longer!
The pattern for the ruffles is basically a doughut shape with a slash through one side. The inner edge (the doughnut hole) is the part that will get sewn to your dress. The outer edge will have the fibers sewn into it.
Create a half circle pattern out of some scrap fabric using the measurements shown. Remember, this is just half the circle -- you're cutting on the fold, so when you unfold the fabric you'll have complete circles.
Fold your ruffle fabric so you can cut multiple ruffles all at once. A good way to do this is to wrap your fabric around a piece of cardboard that's about the same radius as your doughnut, then slip the cardboard out and voila! Perfectly aligned folds. Pin the pattern to the pile of fabric and cut.
Cut along one of the folded edges to make an open circle, and then tidy up the edges to get rid of any corners.
If your fabric wants to ravel, finish the inner edge of each ruffle with a zigzag stitch or a serger.
Preparing the Fiber
Slice the plastic tube and pull the fibers out. This is much easier said than done, but a thread ripper and an extra pair of hands will get you there.
Decide how many fibers you want to put in each ruffle. I wanted maximum brightness, so each of my ruffles has 3 fibers sewn into it. This is the most that would fit into my cording presser foot's hole, so that made it easy to decide.
Try passing two or three fibers through your machine's presser foot to see what your options are.
Since I used 3 fibers in each ruffle, I separated out 3 fibers from the bundle.
Next, we want to add curl to the fibers.
Find a mailing tube or paper towel roll, or something else heat-resistant with a pleasing diameter. Wrap your 3 fibers around in a spiral pattern, making sure they don't cross each other.
Gently heat the whole tube with a heat gun. You'll see the fibers react and crinkle up a bit when they get hot. Be careful not to heat them so much that they melt or break, but get them nice and evenly hot. Let them cool COMPLETELY before you remove them from the tube, and they'll hold their delightfully bouncy spiral shape.
Note: You may find yourself wanting to try this with all 84 fibers at once instead of just 3 fibers at a time. It seems like a good idea, but it isn't. If the fibers get too hot and fuse together, they'll break when you try to pull them apart, and there's no fixing that! So be patient.
Take your 3 fibers and thread them through the hole in your cording presser foot.
Set your machine to a loose zigzag stitch (or whatever is recommended in your manual). Be sure to make the stitch length fairly long. A very short stitch length will cover up the fibers and not let the light come through. You want enough stitches-per-inch to secure the fiber but not so many that the light gets blocked by too much thread.
Make sure you have 5-6 inches of fiber pulled through the presser foot, so you have some "lead" for attaching the lights. Stitch along the outer edge of your ruffles, pulling gently on the fibers as you sew. Your fibers will end up encased in a zigzag thread "tunnel".
Repeat with each ruffle.
I have 30 ruffles on my skirt. This may get a little tedious. You may want a good audiobook for this portion. I recommend Name of The Wind by Pat Rothfuss.
Stretch the ruffles out with the extra loose fiber at the top, and pin them to your dress, spacing them artfully.
Once all the ruffles are done and pinned, sew them in place.