Measure where and how you want the bandana to sit on your dog's neck. This isn't necessarily the same size as the collar. For example, my dog's collar is 14", but I use size medium, and the 16" or 17" snap placements, because I like a loose fit look.

If your pupper is extra floofy, consider sizing up

Size up if between sizes

If you want tie-on styling (as shown in video below), go up at least one size