Plan the exposed width and ends
Choose the finished width you want to see beyond the collar seam. The strip's cut width must include that exposed width, the allowance used to attach it and the fabric consumed by its outer-edge hem. This example uses a single-layer strip with a narrow hem.
Single-layer strip width
Add the visible ruffle width, attachment allowance and fabric used by the hem. Choose each value before cutting.
For length, measure a short section of the collar's outer stitching line where the ruffle attaches. If that target section is 10 cm, compare test strips of 15 cm and 20 cm gathered down to 10 cm. Record the prepared strip length that gives the fullness you want; these trial lengths are not a universal ratio. Thick cloth can produce excessive bulk with the same ratio that looks light in fine cotton.
Mark where the ruffle ends on each side. Finish any exposed short ends before gathering and keep those finished ends clear of the collar's later neckline seam. Write down the chosen hem, attachment allowance and end treatment before cutting the full strip.
Prepare the strip and matching marks
Finish the long outer edge using the tested hem. Mark corresponding sections on the strip and collar so you can distribute gathers without pushing them all toward one end. Check that paired collar sides receive equal prepared lengths where the design is symmetrical.
Use a marking method that will not stain or leave permanent holes. On delicate fabric, test both the gathering stitches and their removal on a scrap.
Gather and baste to one collar layer
For this sample, place one long gathering row in the seam allowance and one just beyond the permanent seam line, with thread tails left free. Test removal on the same fabric first; the outside row must come out cleanly without lasting needle holes. If it does not, place both temporary rows inside an allowance wide enough to hold them and test the gathering again.
Two temporary gathering rows
Blue dashed rows are temporary. The clay line is the permanent join. Remove row 2 from the visible face after sewing, checking first that the needle holes disappear.
Collar sandwich: edge section
Layers are separated here so you can read them. Both collar right sides face inward, toward the ruffle. Match their outer raw edges at the left. The finished ruffle edge points inward; keep the neckline edge open for turning.
Draw up the threads in sections, matching the strip marks to the collar marks. Baste the raw gathered edge to the right side of one collar layer, with the finished ruffle edge pointing inward across that layer. Spread the gathers and check both ends before sewing the second layer over it.
Enclose the gathered edge
Place the other collar layer right sides together over the first. Keep the ruffle between them and clear of the stitching except along its gathered attachment edge. Sew the designated outer collar edges, leaving the neckline attachment edge open.
Turn through that open edge and inspect before reducing any bulky allowances further. Remove gathering threads visible on the right side, then press without flattening the ruffle. The American Sewing Guild gathering guide explains section matching and removing visible basting.
Attach the completed collar to the garment using the flat-collar pattern's neckline and facing instructions.
Frequently asked questions
Why is one side fuller?
Compare the prepared strip lengths and corresponding pattern match points. Redistribute the gathers within each marked section while the join is still basted.
Should I leave both gathering rows in place?
Remove any temporary row visible on the finished face. Check the remaining threads do not pull the seam or obstruct the intended finish.