An i-cord is a simple knitted tube. 

With DPN 

Cast on 3. Slide stitches to opposite side of needle. The yarn should be coming out of the left stitch. Pull the yarn from behind. *K3. Slide stitches across needle. Repeat from * until desired length. To bind off, thread yarn through large-eyed blunt needle and pull it through all three stitches 2 or 3 times.