SSSK is the abbreviation for slip, slip, slip, knit, which is a double decrease, and is the mirror decrease of the k3tog. There are multiple ways to work an sssk, which method you choose is up to you. Additionally, take into account how well it matches your k3tog (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 and third stitches, then put the left needle into the three slipped stitches in front of the right needle so the three stitches can be k3tog.
Slip first, second, and third stitches purl wise, then put the left needle into the three slipped stitches in front of the right needle so the three stitches can be k3tog. This basically is a k3tog-b or k3tog-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 2 stitches and work a k3tog. Be careful not to stretch the first stitch.
Method Four (combo)
Slip the first stitch from the left needle to the right needle as if to knit and repeat so there are two unworked stitches on the right needle then slip a stitch to the right needle as to purl. Put the left needle through the fronts of the unworked stitches going in from the left and keeping the left needle in front, then knit all stitches together.