Knitting two socks on two circular needles at the same time is a bit of knitterly magic that, like so many other things in knitting, seems a lot more complicated than it is. The first time you knit two socks at the same time you're likely to be confused some of the time, but trust that everything will work out and soon you may be wondering why you ever knit socks one at a time.
This tutorial is designed to make that learning curve a little easier because you're not knitting socks for any particular foot (though these probably would fit a toddler); they're just for practice. And the larger weight yarn makes it easier to see what you're doing and understand what's going on than working in a smaller scale might allow.
To get started, you'll need two circular knitting needles of the same size. They can be the same length or different lengths if that makes it easier for you to tell which needle you're working with; I like to use two needles that aren't exactly the same so I can always tell which two ends I'm supposed to be working with.
These instructions will help you get through any socks two at a time, but if you want to follow along you'll need about 40 yards of medium weight yarn and two needles in the US 7 to 9 range (that's 4.5 to 5.5 mm). You'll also probably want at least one stitch marker or a safety pin, a yarn needle or crochet hook and scissors for finishing.
Start with your yarn wound into two balls; each sock will be worked with a different ball of yarn. Pick a needle and cast on for the first sock. If you're following along, that would be 20 stitches.