SSK (slip, slip, knit) is a mirror image of K2tog, decreasing your total number of stitches by one.
How to work a SSK decrease
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :
K1, SSK, knit to end. (9 sts)
I have knit the first stitch of the row and am now ready to work SSK:
Step 1 – Slip 1 stitch knitwise from the left hand needle to the right hand needle, slip the next stitch on the left hand needle to the right hand needle in the same way as the previous stitch.
Step 2 – Place the tip of the left hand needle through the front loops of the two stitches that were slipped onto the right hand needle.
Step 3 – Pass the yarn around the back, up and over the front of the right hand needle (as you would if you were knitting a normal stitch).
Step 4 – Pull the yarn through the two stitches, slipping them off the left hand needle.
One descrease completed.
On the above sample I have worked a few more rows with a SSK decrease near the start of each knit row. This decrease slants to the left on the knit side of the work.