A slip stitch (abbreviated “sl” in patterns) is worked by passing a stitch from the left needle to the right without wrapping the yarn. It can be slipped knitwise or purlwise depending on how the needle is inserted. In either case, the yarn is held to the back of the work unless stated otherwise.
If the pattern does indicate how to slip the stitch, slip it purlwise.
Slip Purlwise
Insert the right needle from back to front as though to purl. Slip the stitch off of the left needle without working.
Slip Knitwise
Insert the right needle from front to back as though to knit. Slip the stitch off the left needle without working.
