I needed a belly band to put on my 14 year old chihuahua Jose' to prevent him from marking in the house. After housetraining him when he came to us at 11 years old he has been reliably clean in the house for the 3 years that we have had him, but I think there is an unspayed female in heat in our neighbourhood. Jose' comes from a hoarder and was not neutered until just before we got him so he knows what his "boy parts" are for. So time for a belly band to reinforce that all pee belongs outside.
I tried a cotton one that I had made for a little dog that boarded with us but it was uncomfortable for him as he has very little fur on the inside of his backs legs. I decided to try making one out of something soft and stretchy. I choose blue polar fleece for the lining and a patterned cotton t-shirting material for the outer layer. Here is how I made it...
Decide how wide and how long you need you band to be, For Jose' I wanted the finished band to be 4" wide and 14" long. (He is 12" around just in front of his back legs). I cut a strip 4 1/2" X 14 1/4" from each of my 2 fabrics. Remember when you are measuring that these fabrics stretch so take that into account when cutting them out. If you cut them too long the excess can be trimmed of after the velcro is applied.

Place the fabrics right sides together, pin to hold the 2 layers in place and, using a straight stretch stitch or a narrow zigzag, sew around 3 sides (long, short, long) with a 1/4" seam. It is important to use a stretch stitch so the stitches don't break when you stretch the band on you dog. Cut across the corners and trim the polar fleece close to the stiching line to reduce bulk.

Turn right side out and push out the corners with a chopstick or a knitting needle. It will look like this...

Sew the hook side of the velcro to the
sewn end of the wrong side of the band.

Try the band on your dog and mark where to put the loop side of the velcro on the wrong side of the band. Stitch the velcro in place and trim of any extra fabric off. If you wish you can finish the edge that you have trimmed with an overlock or zigzag stitch.

This is what it will look like...

And here is Jose' modeling his "bad boy band"...

He is much more comfortable in this soft stretchy band.