With worsted weight yarn (or sport weight doubled), on approx. (US) size 9 needles,
Cast on 108 sts.
Row 1: Knit across
Row 2: 3 border stitches, ( slip first st and knit the next 2), ( K2 tog. K6, YO, K1, YO, K6, K2 tog. twice, K6, YO, K1, YO, K6, K2 tog), repeat ( ) twice more, K 3 for border
Row 3: Purl every st.
Repeat these three rows over and over until reaching the length you want. Remember the blanket will be much longer after blocking. Keep the border stitches on every row, always slipping the first stitch every row.
You can adapt this to a finer or heavier yarn, smaller or larger size blanket, by changing the number of the pattern repeats used. One pattern repeat contains 34 sts.