If you want to shape a piece of knitting, you will need to increase or decrease the number of stitches on the needle. This will make the piece of knitting wider (increase) or narrower (decrease).
Some patterns use the abbreviation "Inc" for increases, and "dec" for decreases. However, the pattern should also tell you what type of increase or decrease to use, as there are several different methods. Some are (almost) invisible, and others can be decorative.
This tutorial is for the simplest decrease stitch, "knit two together". In most patterns this is abbreviated as k2tog.
How to knit a k2tog decrease
Your pattern will tell you where the decrease will be, so you would knit to the point of the decrease. In this example, I have cast on 12 stitches and knitted 3 rows in stockinette stitch (or stocking stitch). On the next knit row, the instruction is
k1, k2tog, k the remaining stitches (11 sts)
So I knit the first stitch on the left needle, and now I am going to knit the next two stitches together as follows:
STEP 1 - Insert the right-hand needle through the next two stitches on the left needle. Insert the needle from front to back as if you were knitting a normal stitch.
STEP 2 - Pass the yarn under and around the tip of the right needle (as you would when knitting a stitch).
STEP 3 - Pull the new loop on the right-hand needle through the two stitches on the left-hand needle.
STEP 4 - Slip the two stitches off the left-hand needle, and the decrease is completed.
Remember, I had cast on 12 stitches when I started knitting. After the decrease, there are 11 stitches (two on the right needle and nine on the left needle). So the k2tog has decreased the number of stitches by one stitch.
I have continued to decrease at the beginning of each knit row in my sample, and you can see that the edge is sloping where the decreases have been made (on the right hand side of the picture).