The muff is made up of two layers of knitting, the inside in stocking stitch, the outside in reverse stocking stitch with cables. The edges are knit in garter stitch. You will start with the provisional cast on and knit everything in one piece. In the end you fold the inside against the outside and sew the seam, then the cast-on edge and the finishing edge are grafted together in kitchener stitch.
You have the possibility to knit the inside and outside in different yarns, the pattern explains how to do this.