Just lay it flat under you dog, wrap it around the and then clip the straps up around the neck of the legs. My dog is a 17lb bichon.
The straps and buckles look like they would be super durable, but a lot of th seams and stitching only gets the very edge of the straps. Couldn't even test it or try on because the straps were broken.
Large size fits perfect on my 60 lbs labradoodle. We like the velcro patch that reads service dog in training and can replace once fully trained. This is what it looks like on my 6 month old pup.
It fits nicely and doesn't interfere with his movement at all. It fits perfectly and she behaved like my service dog.
It seems durable enough, but there is not much to it. Weight of the buckle makes the harness rotate so the buckle hangs in the middle.