SSK is the abbreviation for slip, slip, knit. This is a single decrease, and is the mirror decrease of the k2tog. There are multiple ways to work an ssk, which method you choose is up to you. Additionally, take into account how well it matches your k2tog (if you’re pairing it with a knit two together).
Slip first stitch to right needle as if to knit (this twists the stitch), slip the second stitch, then put the left needle into the two slipped stitches in front of the right needle so the two stitches can be k2tog.
Slip first and second stitches purl wise, then put the left needle into the two slipped stitches in front of the right needle so the two stitches can be k2tog. This basically is a k2tog-b or k2tog-tbl.
Put right needle into first stitch as to knit and twist the needle to the back and go into the back leg of the next stitch and work a k2tog. Be careful not to stretch the first stitch.
Method Four (combo)
Slip first stitch to right needle as if to knit and slip second stitch as to purl (you now have two unworked stitches on the right needle). Use left needle and go through the fronts of the 2 unworked stitches. Keeping left needle in front, knit those 2 stitches together.
See also: SSSK