I have done this tutorial with contrasting colours to show the two different parts more easily. There is no need to do this if you prefer not to, but if you do, the contrast colour will not show in the finished project.
The basis of this technique is that you are crocheting around a loop that you will pull tight.
First make your loop with your contrast yarn, by laying your yarn out thus:
The area you will be using to do your crochet is the double part.
Slip stitch (US single crochet) into the loop.
Repeat this along the loop
Here is a short video showing one complete chain loop.
Continue until you feel that your pom pom is full enough. Complete the chain loop you are working on and do one extra chain after the slip stitch.
Break yarn and pull through.
Arrange the loop so you can see all the ends clearly
Now pull on the loose ends of the loop to tighten up the circle.
Once tight, knot the two ends.
I also tie of the two ends of the main yarn.
You can now use these ends to attach the pom pom to your project.
I love this pom pom as it looks really good, but is also really robust: it will survive both young children and the washing machine!