How to measure the dog
Measure the neck circumference where the collar sits when the dog is on a leash - i.e. about half the length of the neck and really tight, with hairy dogs it is necessary to work your way through the hair as close to the neck as possible or mat the hair down ("slap") - see photo. Don't be afraid to tighten the meter a bit and don't forget to take your fingers out from under the meter, don't add any extra cm.
A tailor's tape measure is ideal for measuring but if you don't have one, use a string, a shoelace, etc. Then measure their length with a ruler/folding or rollable measure or find a "real online ruler" with the search engine and measure it on the screen.
When measuring the back length, place the dog in its natural stance - you can do it by guiding him with a treat, a toy, etc., find the withers by touch (a point between the shoulder blades; approximately where the spine "bends" between the straight back and the neck). Place the tape beginning there and stretch it to the base of the tail - with long-haired BOC/AUS type dogs it is often much earlier than you would expect according to the hair. The idea of "where a dog would end if it raised its tail vertically up" can help you (see photo).