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You will be doing the same thing even though the video shows work already on the needles. You will be putting the needle in the same place and making the stitch.
Put the cast on stitches in your left hand and empty needle in your right hand.
Take the empty needle and insert it into the first cast on stitch from left to right and then wrap the yarn over the top from left to right and pull the loop through.
Now go look at the video again or see these instructions as well and it should make sense. I don't know which method you are doing so I put both.