Cut a square of fabric so that the diagonal is the same length as your dog’s collar. Fold the fabric, right sides together, on the diagonal and press. Unfold the fabric and leave right side down.
At either end of the diagonal and fold a triangle about 5cm back to the middle so when you refold on the diagonal you get this shape, albeit inside out
sew the raw edges from just below one side fold round to just below the other and then turn right sides out through one of the gaps
done , just thread the collar from gap to gap and it is ready for wearing
if you want a more tailored finish you can run a line of stitching parallel to the top fold to create a channel for the collar
if you have a friend with a posh sewing machine why not add a little monogram or name as here …