Rejoin yarn at bind off

I am following a pattern for a baby bootie and am at a place in the pattern where I am completely stuck! The pattern reads to; Rejoin yarn where you started binding off 21 stitches. Cast on 12 stitches using cable cast-on method, Knit across 12 new stitches and 9 from holder. (21 stitches total) I am totally lost on how to rejoin the yarn in order to be able to cast on the new stitches. Help please! Thank you!

pattern website:

Just leave a tail so you can weave it in later, and caston as usual.

You will be casting on the stitches to make the strap of the bootie. See if you can fold up the bootie as you will when you sew it together and get the picture of how it is going to be a bootie. Right where you began binding off the 21 is where you want to start a strap. Usually when it says “rejoin yarn” it just means start knitting with the new yarn, but in this case I guess it will be a bit different because you don’t just want to start knitting but to cast-on some more stitches. To do this I think you will need to pick up one stitch loop from the spot you want to start. See the video about picking up stitches if you don’t know how to do that. I believe it is in the advanced techniques section. After you have one stitch loop on the needle knit into that loop and pull the resulting loop up onto the needle with the first stitch. See the video section for a cable cast on. After you have 2 stitches you then knit between the stitches, pulling the new loop up each time onto the needle with the others until you have 12. After you have 12 new stitches you then turn and knit back across them and proceed to knit the 9 stitches you have on the holder. I hope this helps get you started.