I make "scrunchies" for my dogs collar with scraps.
Very easy. Measure the current collar lengthwise and width including the buckle width. Double the width plus 1 inch for seam.
Long strip to be 2 1/2 times the length, longer if you like. Double fold both ends and stitch each end to to keep each end from raveling. Fold the length, right sides together, and sew together.
Turn the "scrunchie" right side out and slide the collar through.
Tip: Make a trial piece first, about 6" long to be sure your measurements work. My Bichon has several of these, some holiday fabric. For very small dogs just sew the wrong sides together, Too hard for me to turn right side out. No pictures though.