Slip first stitch. Follow pattern to last stitch. Purl last stitch with slipped first stitch, being careful no?

What does mean when knitting direction say: Slip first stitch. Follow pattern to last stitch. Purl last stitch with slipped first stitch, being careful not to twist the row. This will close the round and decrease one stitch. 136 st. Place ? Do I slip on the first stitch i cast on