Line up your two pieces with the right side facing towards you and the edges aligned.

Insert your needle under the 1st horizontal bar on the left hand piece, then insert it under the 1st horizontal bar on the right hand piece.

Insert your needle under the next horizontal bar on the left hand piece, then insert it under the next horizontal bar on the right hand piece.

Pull the yarn tight as you go to see the seam disappear.

Repeat step 3 to the end of the seam, pulling the yarn tight every few stitches.