To begin your I-cord bind off, knit your piece as described in the pattern. The bind off can be worked on either the front or back side, but I show it on the front for ease of illustration.
Before beginning the bind off, cast on three more stitches at the beginning of the row, with all the stitches on the left-hand needle. And easy way to do this is with the cable cast on.
You can cast on more than three stitches if you like to make a wider cord, or fewer to make a narrower cord.