The slip stitch in crochet can be used anywhere it is needed without changing the shape of the project because the slip stitch has no height. Compare this to the single crochet stitch; the sc adds a height of “one.” Compare this to the double crochet stitch; the dc adds a height of “two.” All other stitches add height as well.
A slip stitch is mainly used to connect other stitches. The slip stitch is intended to join rounds, to seam, and to move the yarn and hook to a different spot without adding height to a row. If a ring is being made, the crocheter will chain a number of stitches and then connect them with a slip stitch (sl st). For example, when decreasing for an armhole, a slip stitch moves the yarn and hook in from the edge of the fabric without building height.
First be sure that the ss is indeed called for. Sl St's are most commonly used to move the yarn inconspicuously to a different spot in the work without intending to work into the stitch at a later time. If certain the sl st is supposed to be executed, take care to work loosely enough when making it so that the hook can be inserted into the stitch on the next row or round.
Making a slip stitch is very simple. Just insert the crochet hook into the next stitch, yarn over, and pull the yarn through both loops. That’s really all there is to it. Where and how well the slip stitch is used is what matters most.
If a join looks sloppy, there is an alternate method of joining. Work to the end of the round, but before joining with a sl st, remove the hook from the loop. Insert the hook into the spot where the join is necessary as if to make a stitch, but then pull up the loop just dropped and continue. If it still doesn’t look right, try it again, inserting the hook into the stitch from back to front, and pulling through the loop.
Slip stitches are wonderful to use when piecing things together. This is done on the wrong side of the fabric. Hold the pieces to be joined with right sides together. Insert the hook all the way through both edge stitches. Yarn over and pull through a loop. **Insert the hook into the next pair of stitches and pull up a loop through both layers of fabric and on throughout the first loop on hook. Repeat from ** until the entire length is joined.
Slip stitch seams can be tight and somewhat unforgiving. Stop and examine the work periodically to insure that an even tension has been maintained. If the seam is too tight, try using a larger hook than the one used on the garment.