Baby poncho question

I am making the beginner baby poncho from the free paterns on this site, the one where you join two rectangles to form the poncho.
:?? What kind of stitch is best to join the pieces, since one edge is a knitted row and the other edge is the border edge of several rows? Kitchener or mattress stitch wouldn’t work, right? (did I mention I was a beginner?!)

Thanks, Lori Ann

Kitchener is used to weave together stitches that are not bound off.

Mattress stitch probably would be good because it’s so invisible.