[Puddinpop]

I am ready to pick up thumb stitches on my mitten. It says Knit sts from holder onto 2 circular needles, but add a stitch over gap of thumb. I realize I have to insert cir. needle in stitch holder and knit, but how do I start using a new yarn to do this? Do I put a stitch on the needle from the new yarn or just start using the yarn without connecting it to anything? I also, don't know where to add an extra stitch. Please help. I don't want to give up.