You will need a fabric tape measure to do this.
Simply measure around your dogs neck, where the collar will sit. You should be able to insert two - four fingers, side-by-side, away from your dog, under the tape to provide enough slack. Smaller dogs may not need this much space, and a bigger dog may need more. It's up to you.
The placing of your fingers is with your hand perpendicular to your dogs neck. Not flat against it.
Whatever size you give me will be the length of the collar from the buckle to the middle hole. This allows two holes either side for adjustment. I have recently changed this guide to be a measurement of just the leather part of the collar. This seems to fit with what many of my customers expect.
Remember that in saddlery a 1/4 inch either way is considered good!
If you have a young dog that is still growing let me know when you order and I will make that the length to the first hole. This allows a lot more room for your dog to grow into their handmade collar.