I just did applied i-cord on a sweater, and this is what I did.
I first picked up stitches (see the video that others have mentioned for picking up stitches) around the entire edge that I was planning to i-cord. One vertical edges I picked up 4 out of every 5 stitches (ie. pick up 4, skip 1, pick up 4, skip 1, etc.) and on curved edges and cast-on/bind-off edges I picked up every stitch. Then, once the stitches were picked up I took a spare dpn (or circular) in the same size and cast on 4 stitches, then slipped those 4 stitches onto the other needle (with all the picked up stitches), the stitch with the working yarn on it will be the 4th stitch from the needle tip. *Start working your i-cord, knitting the first 3 stitches, then do a SSK with the 4th stitch together with the first picked up stitch.
Now slip the 4 stitches on the right needle back onto the left needle and repeat from * until you've used up all the stitches, then bind off your i-cord and you're done.