You measure an existing collar as described above - then you can use the dimensions given for the respective collars.
Please note - the wider the "new" collar, the more leeway
you should plan for the top.
Measure the dog:
In the area of the red and blue lines (see picture below), measure the circumference of the neck tightly. Add 2 cm to the narrower girth.
The dimension calculated in this way should fit within the specified sizes of the collars for sale.
Measure tightly in the area of the yellow line and add 1.50 cm.