Double crochet stitch is one of the most basic crochet stitches. It is very useful in many projects and patterns, from crocheted lace to mittens and sweaters. It gets a reputation of being more difficult than single crochet stitch, but though it is perhaps a bit harder to learn at first you will soon find that it comes naturally. It will probably not take more than an afternoon to learn the technique and to practice it enough to become comfortable with the stitch.
To learn you will just need a ball of yarn and an appropriate size crochet hook. Bulky or worsted yarn is probably best for learning. You'll want to use a size I, J, or K crochet hook to accompany heavy worsted weight to bulky weight yarn. The only other thing you'll need to get started is a basic knowledge of how to create a basic chain.
To practice double crochet, you will first need to make a chain of twenty stitches. Wrap the yarn around the hook, as if to make another chain stitch, but do not pull yarn through the loop on the needle. Skipping three chain stitches, insert the needle into the fourth stitch from the hook. Wrap yarn and pull it through the chain stitch. You will now have three loops on the needle. Wrap yarn again and pull it through the first two of these loops. Two loops remain on the needle. Wrap the yarn one more time and pull it through the remaining two loops. You have completed a double crotchet stitch.
Continue to double crochet until you reach the end of the chain, making one double crochet stitch in each chain stitch. Chain three to turn. This is called the turning chain and counts as the first double crochet stitch in the new row. So skip the first stitch in the row below, inserting the needle into both loops of the second stitch. Create a double crochet as before and continue across the row. At the end of each row work the last double crochet in the turning chain from the previous row. This will ensure that you maintain the same number of stitches in each row.