A stitch holder can be another needle, a large safety pin like holder or a piece of yarn. You won't use those sts until you make the thumb, but you need them to complete the hand part so you CO 6 to fill in the gap. Then later you go back to the held sts and knit them and pick up sts from the cast on ones, so I'd recommend a backward loop or knitted CO for them as it's easier to pick up sts from them.
However, another way to both 'hold' them and 'cast on' new sts is to take a different piece of yarn, only needs to be about 12" or so, and knit only those 6 sts with it. Then slip those sts back to the left needle and reknit them with your working yarn. Then when you do the thumb, pick out the odd yarn carefully and you have 12 sts ready to go.