SSP (slip, slip, purl) is a mirror image of P2tog, decreasing your total number of stitches by one.
How to work a SSP decrease
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next purl row the instructions are :
Purl to the last 3 stitches, SSP, P1. (9 sts)
I have purled to the last 3 stitches of the row and am now ready to work SSP:
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 – Slip the 2 stitches back onto the left hand needle, by inserting the left hand needle from left to right.
Step 3 – Insert the right hand needle through the back loops of the 2 slipped stitches (from left to right).
Step 4 – Pass the yarn around the back, up and over the front of the right hand needle (as you would if you were purling a normal stitch) .
Step 5 – 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 SSP decrease near the end of each purl row. This decrease slants to the left on the knit side of the work.