The same way you do when finishing, except that you have sts left over. Okay, knit across to the first marker, take your other strand of yarn and k 2 sts with it, pull the 1st of those 2 over the 2nd. Knit one more, bind off another st. Knit another, BO another. By now, the sts you knit at the beginning are on the end of the needle, but that's okay. Keep going until you have the center sts bound off, and do one more. You should have an equal number of sts on each side of the gap. Knit across the rest of the row. Turn and do the WS row, dropping the yarn at the gap, and picking up the other yarn. Turn for the RS row, knit to 2 sts before the gap, k2tog, drop the yarn and pick up the other yarn, ssk or slip 1, k 1, psso and knit the rest of the sts. Repeat decreasing 2 more sts on the next RS row the same way.