With the yarn in front, slip the required number of stitches as if you were purling them. Every pattern I've ever seen for this stitch uses five slipped stitches, but if you want to design your own basic smocking pattern, you could slip more or fewer stitches as you like.
Once you've slipped the stitches, complete the strand for the Butterfly Stitch by putting the yarn back to the back of the work. Make sure that your slipped stitches are spread out on the needle, not all bunched up together, or your strand will be short and make the fabric bunch in that part of the work.
As you work across the row, try to make your tension consistent on each strand, as well as on each row where strands are worked.
