Lay the collar out on a flat surface
Measure the collar from buckle to buckle, or from button hole to buckle - as shown in the image below; the measurement should be the space between the red lines shown in the diagram:
Measure the widest part of the collar. In most cases this it the buckle.
This measurement will provide you with the width for the opening you need for the correct Bandana size - please note a slimmer collar may be required to best fit the Bandana.
How to measure the width is shown below; the measurement should be the space between the red lines shown in the diagram: