Similar to working the mattress stitch for stockinette pieces, thread a darning needle with a length of matching yarn no shorter than double of the length of the seam to be sewn.
Align the pieces with right side facing each other.
Beginning at one end, insert the needle under the first horizontal bar of the knit stitch on one side and then the corresponding bar on the other side. Make sure there are two complete knit stitch columns running along the seam on the right side after the seam has been drawn closed.
Repeat the same step until the end. Secure and weave in the ends.
You can find Valerie's videotutorial here.
