Correct measurement - for this you need: A tape measure, your dog and a little patience.
Position your dog and have the measuring tape ready. If he looks scared, praise and treats often help.
Measure around your dog's neck, preferably where the collar sits. Don't hold the measuring tape too loose, but also not too tight.
Measure your dog's chest at the deepest point. Don't hold the measuring tape too loose, but also not too tight. Give your dog praise and treats in between.
Measure the back length of your dog from the base of the neck to the base of the tail. Don't measure neck and tail.
Praise your dog, he did a great job!