This cast on leaves a nice, stretchy edge. It is specific to k1, p1 ribbing.
Begin with a length of waste yarn. Cast on half of the total number of stitches you need. (If you need an odd number of stitches, cast on half + 1 stitch.) Cut the waste yarn, leaving a 6 inch tail.
Change to the working yarn and complete the first row as follows: *Knit 1, yarn over. Repeat from * to last stitch. Knit 1.
Complete the second row as follows: *yarn forward, slip one stitch purlwise, yarn back, knit 1. Repeat from * and slip the final stitch.
Complete the third row as follows: *Knit 1, yarn forward, slip one stitch purlwise, yarn back. Repeat from * and knit the final stitch.
Repeat the second and third rows.
Change to K1, P1 ribbing. On your first row you will begin with a purl stitch. Once you have worked several more rows of ribbing, you can carefully snip the waste yarn and extract it from the cast on edge.
You will be left with a stretchy edge that is well suited to your ribbing.