First step is to use an existing, disposable diaper to create a pattern. Not sure how else to tell you, as that is how I did it & dogs are different sizes. As most of you know, I happen to have a small dog. An excitable, spoiled, miniature long-haired dachshund named Sera. My baby.
As Wal-Mart here no longer has fabric, I used bandana's, plus, I have a ton of them. Now, I'm not the greatest seamstress, too impatient to do it right, I want it right now, but I do get better each time.
So, instead of trying to get outer fabric just right, I cut to approximate size & sewed on, then trimmed to half inch out, so there is room for a seam. I just cut an X in the tail hole.
Once seams are ready, fold in twice & pin in place, then sew it on.
Inside is not too pretty, but you can see the seams & more importantly, it blends in w/ final fabric. Plus it's for pee, so not a big deal on how it looks.
I was also stitching around the inner pad again to help hold in place, but that affects the water proof-ness, (I know, it's not a word, but this is my tutorial, so I can make up all the words I want.)
After all seams are sewn on, it is time for velcro. Need loopy fuzzy side on the butt, mid point between edge & tail hole.
I created tabs from the fabric scraps, made them just a bit widerthan the hooks from velcro. I used a piece of velcro equal to that on the back, cut it in half & then turned those halves sideways for the tabs.
top & bottom, each have 4 inch piece of 1" think elastic. stretched out to 2 inches in on either side.
Again using zigzag stitch, reienforced the ends.
Sides (legs) 4 inch piece of very narrow elastic, maybe 1/4". Not exactly sure on measurements, just eye-balled it.
Here is the finished product. Hope this helps.