I just had to do some sewing with my colorful Halloween fabrics, so I made a dog collar scrunchie for Zelda. Might as well decorate the dog too, right?
I measured the widest part of the plastic buckle, then cut 3 strips of fabric, each about 1.5" wide across the whole width of 45" wide fabrics. I wanted it to be pretty close to the size of the actual collar so it wouldn't be too floppy.
I sewed those together on the long sides and then hemmed the ends, then I sewed the final seam to make it up into a tube and turned it right side out. Next I just fed the collar in and scrunched it up.