S2tog-K1-p2sso is a double decrease, meaning you remove 2 stitches from your stitches count on your needles. The decreased stitches (from both sides) “hide” behind the central stitch.
Work according to pattern until you need to make a S2tog-K1-p2sso decrease.
Slip the 1st two stitches together, as if knitting them together.
Knit the next stitch.
Pass the tip of the left needle into the 2 slipped stitches.
Pass the 2 slipped stitches over the knitted stitch.