Cast on two stitches. K2 in each st (4 st on needle).
Turn. K2 in first st, K2, K2 in last st.
Continue in garter st increasing at the beginning and end of each row until you almost run out of yarn.
Cast off. (You need enough yarn left to equal 4 times the length of the top of the scarf in order to cast off). This makes a shallow triangle. I often sew on a large bead pendant to each end and then you can just flip the ends over without tying.