Cast on 2 stitches.
Knit those two stitches.
Increase every row by doing:
KFB, knit remaining stitches
On the last increasing row, K2tog on the final two stitches of that row.
Decrease every row by doing:
Knit all stitches, then K2tog at end of each row.
Knit the final two stitches together.
Weave in ends.