Cast on 20 stitches (any method). Knit across, then turn and knit back. Repeat until you have 20 ridges or 40 rows then cast off.
Variations
Scarf
Cast on 40 stitches (more or less according to taste, use your sample to judge width). Knit plain or garter stitch until both ends of the scarf will reach to the waist when draped over the neck. This is what I consider minimum length. Continue until you are tired of it or run out of yarn! At the end, cast off. If you want a fringe, cut short lengths of yarn--twice as long as you want the fringe--and knot through the cast on and cast off stitches with a crochet hook.
Hat
Cast on about 60 stitches and knit in garter stitch until the work reaches around the head. Cast off and sew the cast on edge to the cast off edge stitch for stitch. Gather one selvedge edge to close the top. Add a pompom if desired and turn up edge for a double layer over the ears.
Other garter stitch projects
Elizabeth Zimmerman patterns--modular Hooded jacket: Newsletter #7, or in Knitting Without Tears. Sweaters in Wool Gathering numbers 3, 9, 17, 21, 19, 23, 27.
Garter stitch makes great shawls since it lays flat.