Thank you, Practical Crocheter. I think that was a really good explanation. I just wanted to make two comments about my experiences learning foundation double crochet.
1. Until I got really used to using it, my foundation stitch row came out with a larger gauge than the rest of my project. So when you practice it, practice until your fdc's have the same gauge as your normal double crochet in the same yarn.
2. At first it can be difficult to keep track of that chain stitch you make in the beginning of the stitch. Until you learn to see it (and to help with learning to see it), hold on to that chain from the time you make it until you insert the hook into it to start the next stitch.