The first two images show period examples of the frilled fillet. The one on the left is from the Maciejowski Bible. The other is from the Manesse Codex.
Step 1: Cut out two lengths of linen with a length equal to the size of your head (around your forehead) plus some for seam allowance, and a width of about 2.5". If your linen is lightweight (which mine is) cut a third piece of equal size. I used a piece of cotton as it will be completely hidden and I don't want to waste the linen.
Step 2: Pile the strips together, with the inner layer on top, and sew them together along one edge with about a 1/4" seam allowance. This is the bottom edge.
Step 3: Cut out one length of linen about 2.75" wide and equal to about 1.5 x the length of the first strips. The longer you make this strip, the more pleats will be in the frill. Fold in half lengthwise and sew together with about a 1/4" seam allowance. Do NOT turn inside out- you'll need that seam allowance to attach it to the band.
Step 4: Put the three-layer thick band and the frill down on a flat surface, and pin the ends of the frill to the ends of the band. You'll need to pin the frill between the layers and fold the top layer seam allowance in while you do. I put the front outer layer and the inner layer down at the back, so I was looking at what would be the back of the band. Only pin the frill to the back layer- leave the front two free.
Step 5: Find the center of the frill and pin down to the center of the band. Take the center of the frill to the left of the pin and pin to the center of the band between the end and pin. Apply that same thinking all the way across, creating even folds as much as possible until the entire frill is pinned to the band. Sew the frill to the back band layer (as close to the edge of the fold on the back layer as you can).
Step 6: Turn the band over and fold the front two layers of the band down along the top to (same as you did at the back) and pin down. You won't be able to machine sew the front to the frill because you don't want the stitching to show at the front, so get ready to hand sew.
Step 7: Hand stitch the top edge of the front hem fold to the frill in a tight stitch, making sure that the thread isn't overtly visible. This step will take the longest to complete, depending on how fast you hand stitch.
Step 8: Fold in half, putting the front in, and sew the ends together (double check your length to make sure you don't use too much overlap and make it too small). Turn right-side out.
Step 9: Determine how you would like to finish the ends. If you have enough seam allowance to turn under, do so by hand stitching (don't sew through to the front). I didn't end up having enough, so I added a panel to cover the open ends, and hand stitched it in place.
Step 10: Remove all pins and admire the fruits of your labor! This took me the better part of a day (about 5 hours all told), but by combining machine sewing and hand sewing, I saved time without sacrificing the finished look.
This fillet is for my mom, so it's a little larger than mine would be. To complete the style, you'll need a barbette. Stay tuned for that how to (which is a lot easier)! You'll also need a caul (or hairnet) to polish the look.
On a completely different note, I've decided not to enter the A&S50 Challenge. The fact is, I've already got a pretty full project pile, and I need to be working toward completing it without feeling that I have to add more to it.