Even though I've never personally done this, what you're doing is called provisional cast on. These 7 stitches will later be taken out (the 7 stitches with the waste yarn) and you'll be working with your actual yarn. You'll just need to be very careful as you rip these stitches out, doing it slowly, one stitch at a time, and the loops you'll pick up will be your "live" stitches. Just take a scrap piece of yard, cast on 7 stitches and cut the yarn, then slip them back onto your left needle and pick up your working yarn and begin knitting. Is this clear as mud??