step one: in
Insert the right-hand needle into the first loop on the left needle from right to left in front of the left-hand needle.

step two: around
Wrap the yarn around this recently-inserted needle from left to right on the top of the needle.

step three: through
Pull the loop you just made [with your wrap] through the original loop on the left-hand needle.

step four: off
Slide the original loop off the left-hand needle.

step five: done!
One purl stitch completed!