How to hand knit a no-curl edge for the plain stockinette stitch
The basic hand knit stockinet stitch always rolls and curls on the left and right sides and somewhat curling on the bottom and top edges. You can solve the problem by knit a border with a non-rolling stitch such as a garter stitch, seed stitch, or ribbing stitch. If you want a more subtle stitch change but reduce the curling tendency, I find the slip stitch a good choice,
In the video below, I show the alternating of the knit and slip stitch on the front side and purl stitches on the back side. Cast on an odd number of stitches if you want to have symmetrical borders continuing from the bottom edge. Pick any number of rows and side stitches for the border. More edge stitches and rows make the fabric sturdy and flat.
I hope this method gives you one more choice for knitting the plain stockinette stitch.