To make a purl stitch bring the yarn to the front, see figure 2, of the work and insert the right needle into the first stitch on the left needle from back to the front as shown in figure 3.
Then wrap the yarn around the needle from the back to the front as shown in figure 4.
Finally pull yarn through the stitch and leave the resulting loop on the right hand needle and slide the loop off of the left hand needle as seen in figure 5.
To get used to this stitch practice on as many rows as necessary to get comfortable with the technique. Once you are comfortable you can try alternating knit stitches and purl stitches within a row to get a fabric that draws together and is called ribbing seen in figure 6. Just remember to move your yarn from the front of the needles when purling to the back of the needles when knitting.