With I hook Chain 32
(I came up with this chain by measuring the chest of my pup. Measure your pups neck and add or subtract accordingly. My pup has a chest of 8 inches.)
Ch 2
HDC in second ch from hook and across.
Ch 2, turn
*Hdc in next 2 hdc, 2hdc in next hdc* repeat across.
Ch 2, turn
Hdc across.
Ch 2 turn
*hdc in next 2 hdc, 2 hdc in next hdc* repeat across.
Ch 2 turn
Hdc across.
(repeat the above pattern if your dog has a longer chest.)
Ch 2 turn
Hdc in next 7 hdc, ch 5, skip 5 hdc, hdc in next 30 hdc, ch 5, skip 5, hdc in next 7 hdc.

(this row will make the holes for the front feet. This one fits my pups which have legs of 4". Adjust this to fit your dog by increasing the ch # and the number
skipped as well as the # at the beginning and ending (I.e. hdc in next 10 instead of 7.) I found out the number approx by measuring between my pups front legs.)

Ch 2 turn
Hdc across to 5 hdc chain space, 5 hdc in ch 5 sp, hdc across to next ch 5 sp, hdc in ch 5 sp, hdc to end.
Ch 2 turn
Hdc in next 2 hdc, 2 hdc in same sp, *hdc in next 3 hdc, 2 hdc in next hdc* repeat to end
*Ch2 turn

Hdc across* 4xs

Forming back
Change to H hook
Ch 2, turn
Hdc 17, Change to I hook, Hdc 31, Change to H hook, hdc 17
Change to G hook
Ch 2 turn
Hdc 17, Change to I hook, Hdc 31, Change to G hook, hdc to end
Ch 2, turn

Change to F hook
Hdc 17, Change to I hook, hdc 31, Change to F hook, hdc to end
Fasten off

Use I hook

Begin 18 Stitches in (where back begins to lengthen) and join with slip stitch
Ch 2
Hdc 28
*Ch 2 turn
Hdc across* repeat 3 times or as many as needed to make length (should be one ½ to one inch from tail)

Change to H hook
Ch 2 turn
Hdc in first 2 hdc, *dec in next 2 hdc, hdc in next 3 hdc* repeat across
Fasten off

Back complete
Back of neck:

With I hook
Counting from one end of beginning chain, count 10
Join with slip stitch
*Ch 2

Hdc in next 12 stitches. Turn* 4 times (or for as long as u wish neck line to be. This was long enough for my pups)

Legs:
With J hook
Join with slip stitch in any area of the leg hole
Ch 2
Work 16 hdc in leg hole area.
*CH 2. Turn
Hdc in each hdc around. * repeat for rounds needed to make one inch.
Took 4 rounds to make one inch for mine.

Finishing.
Right sides together, stitch together tummy side of sweater.
For nicer finishing, also stitch around each edge with contrasting color in a hdc.

I made a bow and sewed it to the back of the sweater for decoration.