This is the view from the top.
This is the under-belly. It is not seedy. (hee)
Here's how I did it. Bear in mind that (a) there has been no technical editing and (b)the dog hasn't tried it on yet.
This was knit for a 3-4 lb Chihuahua. For other dogs, please scale up with actual measurements from the dog.
CO 20 stitches, K 1 row.
For the next 10 rows, K2 YO K to end. (30 stitches, garter stitch)
Switch to stockinette:
WS: K2 YO Purl to last 2, K2.
RS: K2 YO K to end.
Do these two until there are 40 stitches.
No increases, but keep the edges.
WS: K2 YO purl to the last 4, P2tog, K2
RS: K2 YO k to the last 4, K2tog, K2
Do this until piece measures about 5" (or until you get to between the dog's armpits and waist. )
At the end of a RS row, CO 10 stitches (or enough to make a belt/waistband).
Join in the round or not (could add a button/button hole for this).
What I did was, while knitting in the round:
over the 10 CO stitches, I knit K2P2 rib and stockinette around the rest, for about 4 rounds, then plain stockinette for about 4 rounds, then back to the K2P2 rib around the belly section, stockinette around the rest for 4 rounds. Then cast off the 10 stitches and go back to back and forth stitches.
Another option would be to knit flat and use a button: Cast on enough stitches to go around the belly and a button hole.
Knit the underbelly part in garter stitch and knit a buttonhole into it, maintaining the 2 stitch garter border on the non-cast on side.
RS: K2 K2tog K to end.
WS: K2 K2tog, purl to last 2, k2
Do these until you get down to 30 stitches.
At the end of a row, CO 20 stitches (or long enough for your dog).
Knit in garter stitch 3 rows (or in the right proportion for your dog)
Starting from the last cast on stitch, K5, K2tog YO, K3 K2tog YO, K3 K2tog YO, K to end.
Knit in garter stitch 2 rows
Weave in ends and then sew button(s) on where needed. (I tried to do it in the other order and accidentally sewed through the yarn tail, making it so I had to do it twice.)