This is my tutorial for how to make a patchwork dog jacket : http://adventurousquilter.blogspot.com/2014/01/dog-jacket-tutorial.html
I used that method but because these coats aren't patchwork I didn't quilt the layers together, the only thing that holds the three layers (lining, batting and outer) together is the inner edge stitching and top stitching then the strapping is sewn on through all three layers. So these are super quick to make!
I have found it hard to get strapping in the sewing section of Walmart lately and it was too expensive anywhere else... So necessity is the mother of invention, I figured in the car section of Walmart they'd have tie downs made from the same strapping and THEY DO! Yahoo. This tie down cost about $3.50 and its 6 foot. By the time I cut the hooks etc off I had 60" which is enough for 2 small or medium dog coats. That is cheaper than the strapping was in the sewing section!!!
Then it's a simple job of sewing the 3 layers together; I used flannel for the lining and the outer fabric and a cotton batting scrap (the batting is narrow strips I trimmed from quilt sandwiches joined which works fine)
Add your strap, velcro and the elastic at the front and it's ready to go!
They are like walking Christmas decorations LOL