I used poor George as a live mannequin.
Chain stitch 30 links for the neck. I used a single and a half crochet for the rest.
Crochet around five times and then start adding 2 -4 extra links per circle (especially if you have a bigger dog.) Use your puppy as your mannequin.
Once you get to the shoulders, stop circling. and measure (according to your dog) stopping at each shoulder and then crochet back and forth 5 lines or so (check your dog)
After you are past the shoulders then continue circling until you have a nice long sweater subtracting stitches per line so that it is fairly snug around the abdominal.
Go back to your section in the middle and connect in the center about 10 stitches back and forth until you can connect to the abdominal section.
Now you have holes for the front legs and if you wish can crochet around them for sleeves.