Make one loop as you would for simple loop cast on or make a slip knot on the left hand needle. Knit the loop but instead of slipping the stitch off the left needle insert the left needle into the new loop on the right needle from right to left and slip that new loop onto the left needle. You now have 2 stitches repeat this process until you have the required number of stitches.
This produces a flat, stretchy, loopy cast on which is ideal if you are going to make a picot edge and need to fold the cast on into a hem. You can then either knit the hem up or sew it up. Keeps it nice and flat and saves your work from getting puckered up.