There are a ton of tutorials online, and here is mine:
Here's what you need:
2 double pointed needles
Note: Make sure you pick a needle size that works with your yarn. If you are unsure what to pick, I suggest going with the size suggested on your yarn's label. A general rule of thumb is that your knitting will shrink by about a third, but you should do a tester first, or felt them more than once.
For the 1/3 rule, your i-cord, after felting, will be 2/3 of its original size (you can multiply your desired length by 1.5). So if you want a cord that's 8 inches long after felting, you'll need to knit a cord that's 12 inches.
- Start by casting on a small number of stitches. The more stitches you cast on, the thicker your i-cord will be. I recommend 3-6 stitches.*
- Normally, when you end a row, you turn your needle and begin knitting (or purling) the next row. Here, however, you will slide your work from the left end of the needle to the right (if you are a left-handed knitter, it's right to left).
- To make the first stitch of the new row, insert your needle as normal for a knitting stitch and bring the yarn behind your knitting.
(For a right-handed knitter, this means bringing your yarn from the right side behind and over to the left. For a left-handed knitter, it will be from the left to the right.)
- Pull the yarn tight, and it will close the tube. Finish the row, knitting normally, and repeat steps 2-4 until you read the desired length. Cast off!
*If you have yarn that breaks easily (such as a single-ply lopi), cast on a smaller number. The farther you have to pull the yarn (See step #4: the longer your row is, the farther you will have to pull it), the more likely it will break!