Please understand it has been a very long time since I have done this. Our late dog, Suzy, loved clothes. I put her up on the sewing table and fashioned a rain coat pattern for her using medical paper (the paper you lay on at the doctor's office), she was so sweet and cooperative. She knew it was something for her to wear.

Buy some Swedish Tracing Paper, it is used for pattern making, it is fairly soft and sewable. The Swedish Tracing Paper should be easier to work with than medical paper. Then drape, cut, and shape it on your dog until you have what you want for a pattern.

I hope this helps.

Here is a link that might be useful: Swedish Tracing Paper