Start your Butterfly Stitch by casting on in multiples of 10 + 9 stitches.
The first row begins by knitting two, you then enter a repeating pattern of slipping five stitches purlwise (with the yarn forward) and then knitting five. Repeat this until you reach the last seven stitches and slip five stitches purlwise (with the yarn forward), finishing by knitting two. For the second row (and all wrong side/even rows) purl all the stitches. Repeat this alternating pattern from row one through to and including row eight.
The ninth row begins by knitting four then start a repeating pattern with knitting one under the loose strands from the first eight rows, then knit nine more stitches. Repeat this pattern until you reach the last five stitches, where you will again knit one under the loose strands and finish by knitting four.
The eleventh row starts with knitting seven stitches, and then enters the repeat of slipping five stitches purlwise (with the yarn in front), then knitting five. Keep repeating this pattern until you reach the last two stitches, and knit two. Repeat the eleventh row for rows thirteen, fifteen and seventeen.
Begin the nineteenth row by knitting nine stitches, again entering a repeating pattern by knitting the first stitch under the four loose strands created by the previous eight rows, follow this by knitting another nine stitches. Keep repeating this until you reach the end of the row. The twentieth row, as with all wrong side rows should consist entirely of purl stitches.
Keep repeating rows 1-20 until you reach the desired length.