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Though you are accustomed to using quality cotton threads for patchwork, clothing construction is different. It's still fine to use cottons for any patchwork in your garment pieces, but to actually construct garments, you need a good quality polyester thread. You can find less expensive store-brand threads, but my experience is that those are fragile and do not survive use. Use a 50-wt. thread in a color that matches or blends with your fabric.
When you recycle necktie fabrics into clothing, consider how you want the finished project to drape. If the garment should drape softly like a skirt, dress, or blouse, then you probably do not want a foundation or stabilizer. You may need to stabilize the tie pieces with a narrow strip of very light-weight flexible interfacing or stabilizer over the seam lines to avoid stretching the ties as you sew them together.