I think it would have helped to do the cable cast on. I made a miniture or fake bootie, just starting with the 39 stitches you have after row 19 and doing what it says from there. I took pictures of what I did, but it looks like you are doing well, you just need to try the cable cast on. What Sue says about knitting through the back loop is probably a good idea too.
Here's what I learned from my experiment. When you are ready to rejoin the yarn to add the stitches, put the stitches that were on the holder on a needle so that you can work on the end that is at the edge of where you bound off the stitches. Hold the needle with the new nine stitches in your left hand. Insert the RH ndl (right hand needle) between the first and second stitches at the working end of the LH ndl. Yarn around and draw up a loop. To position the new loop on the LH ndl pull it up with the RH ndl tip, rotating the RH ndl tip away from yourself and down (or whatever it takes) so that you can insert the LH ndl into the loop from the front (like you were purling it). This twists the loop a bit as it goes onto the LH ndl. Then before I snug the new stitch up I insert the RH ndl between the new loop and the next stitch on the needle (doing it then makes it easier to get in). Keep pulling up new stitches until you have added your 12 and have a total of 21 again. (With the cable cast on you are knitting the new loops up working between stitches instead of into the end stitch like on a knitted cast on.)
Then knit back across the 12 new stitches and you can knit into the back loop of the first of the 9 original stitches to tighten the slight gap even more if you'd like.
If you still don't get the cable cast on, I could try posting my pictures of me doing it, but I don't know if they would be any improvement over the video.