Help with Stitching up the Ribbed section of dog sweater

I just finished my first doggie sweater for my little girl and the directions say to start sewing at the neck part which happens to be done in k2 p2.

I'm assuming I am supposed to use the mattress stitching but everywhere I look for help in sewing it up only says how to do it with stockinette stitch or reverse stock...

What do I do? Help meeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[Ingrid]

On the knit ribs, do it as if you were doing st st and on the purl ribs do it as if you were doing rev st st. I'm not sure if it will be perfect, but that's how I'd try it.