Dog sweater

I am making a dog sweater and am stumped on the instructions to sew the Underpiece to the Back ----cannot see how that is going to work - where am I sewing it? The Underpiece measures 3" at the top and the Back measures 8". Just not sure how to do it or where to even start. Please help!

Welcome to KnittingHelp!
Which pattern are you making? Can you give us a name or a pattern link please?

Here is one of the patterns:


The underpiece is sewn to the back at the sides. The top 3" of the underpiece and 8" of the back are the head opening. The instructions have you seam along the 2 side edges leaving an opening on each side for the legs. It’s the left and right edges in the schematics that are seamed.