I have both a graph cross stitch pattern and in-depth video tutorial to help you with this technique but it is really quite simple - cross stitch using a tapestry needle over your single crochet stitches! Now I'd like to offer a little written explanation
This is how you get really smooth, lovely satin stitches. Outline in split or chain stitch, fill in the area with chain or long & short stitches, and then do the satin stitch. The underlying stitches support the satin stitch and help it to lay smoothly.