Grab a collared shirt that you already own (Sigl used her boyfriend’s). Fold the collar in half and mark the middle/halfway point with pins.
Place your paper on top of the collar, and with your pencil roughly trace around the collar edges. If you can’t see through your paper feel with your fingers and mark as best you can. Only trace up to the halfway point where you placed the pins (the halfway point will be your center-back.)
Note: You will want to do this for the top collar (part that folds down) and the bottom collar (what connects folding part to the shirt.) Make sure to mark where the top collar ends on the bottom collar.
Now with your ruler, connect the lines smoothly. For the rounded parts, just connect with a steady hand or you can use a rounded ruler. Add a 1/2-inch seam allowance like above. Then cut out your pattern from the paper. You’ll want to make sure they align properly, so adjust if they don’t.
Fold your fabric in half and pin the pattern pieces on top, aligning the center-back of your pattern to the fabric fold (remember that the center-back was the halfway point of your collar.) You’ll need two fabric pieces of the top collar, two fabric pieces of the bottom collar, and one of each cut in interfacing.
Iron the interfacing onto one of the top collars and one of the bottom collars.
Take the two pieces of the top collar and place fashion fabric to fashion fabric. Sew the collar around the edges at 1/2-inch seam allowance. Then notch (to but not through the stitch line) around the edges, and cut the seam allowance to 1/4 to 1/8 of an inch.
Flip right side out and press with an iron. The piece with the interfacing will be the bottom of the collar when it’s flipped down.
Now sandwich the top collar between the two bottom collar pieces. The fashion fabric should be on the inside. Pin the edges and sew with a 1/2-inch seam allowance. When done sewing, notch and trim seam allowance like you did previously with the top collar.
Flip the bottom collar down and press.
Press the bottom edges of the bottom collar inward 1/2 inch. Then cut the seam allowance a tiny bit to reduce bulk. After you are done cutting, press again, leaving the bottom edges flipped up/inwards at the original crease line.
Pin and sew the bottom with a 1/4-inch seam allowance.
Now the basic structure of your detachable collar is finished. Fit the collar around your neck and mark with a pin where that snap buttons will be. Sew the buttons. At this point you’re all done! You can add embellishments if you want; Sigl glued some lace flowers onto hers.