Crocheting curls is very easy - don't know if there is a term for, in German we called it Korkenzieher = corkscrew, I'll call it curls for now. All you need is a foundation chain. This should be about twice as long as the curl should be when it's ready. Now simply sc 2-4 in each chain. The more stitches you crochet in one chain, the tighter the curls and the more voluminous - but it also becomes a little shorter. The other way round, if you crochet fewer stitches per chain, it curls less and becomes smaller and longer in all.
If you use larger stitches (half double or double crochets) instead of single crochets, the curls become even more voluminous and also a little longer. If you want to have extra thickness, you should crochet 4 double crochets or even treble crochets per stitch.
Update: Meanwhile I have noticed that not only the number of stitches per chain is important for how much the curls curl, but also how tightly or loosely the chains have been crocheted. It curls tighter, the tighter the chains are crocheted, and vice versa, of course, the looser, the less tight the curls. I hope, it's understandable.
All three curls in the picture above are the result of a foundation chain, as it lies on the far left. All were crocheted with half double crochets. The difference: two half double crochets per stitch were crocheted on the left, three per stitch in the middle and four half double crochets per stitch on the right. You can see the difference clearly.
In this picture I crochet two stitches per chain, single crochets on the left, half double crochets in the middle, double crochets on the right. You can see that the curls with two double crochets per stitch are not so nice and evenly, so it is better to crochet four double crochets in each stitch.
In this picture we have two single crochets per stitch on the left, three half double crochets per stitch in the middle and four double crochets per stitch on the right.
Don't forget always to skip chains before the first stitch - skip one chain for single crochets, two chains for half double crochets and three chains for double crochets. At the end I make a slip stitch in the first chain and knot the ends.
Have fun crocheting :o)