Ready to bring the artist out in you!
I'll confess I haven't done this yet, though I do have several never-used ties that obviously do not need to be washed, so won't be exposed to a color fixer such as Retayne. Some thoughts: The polyester ties do not need to be prewashed, as the ties will not bleed color is the finished project is washed. Silk ties are another story, as dyes in silk ties can bleed. You can test small pieces of the ties by soaking in water to see if the dye bleeds. If not, you can use them safely. If they bleed, you can soak with a small amount of dye fixer in water, then dry before use. Or you can dry-clean the finished project.
I learned to make a flat-fell seam when I sewed pajamas for my siblings many years ago. This is an easy way to sew a seam that conceals all raw edges - it is common in many shirts. The seam has 3 main steps: 1) Place the fabrics wrong sides together and sew a 5/8" seam allowance. 2) Trim only one of the seam allowances to about 1/4" - 3/8". 3) Press the remaining seam allowance over the trimmed, turn the raw edge under, and stitch on the fold to finish the seam.