Choose a needle with a long enough cord to comfortably accommodate all of your stitches and also leave room on each side to not have a large gap between the two sides. For many this will be at least 32 inches for the average circumference sock.
With a long enough cable you can even knit smaller items (socks, mittens, sleeves) two-at-time.
You are going to want to have a needle with a flexible cable and smooth join. I have had much success with Knit Picks fixed needles when knitting socks, mittens and sleeves using magic loop. The interchangeable set only goes down to a US 4, so for most socks with fingering weight yarn, you will need to use fixed circulars.
NOTE: this tutorial will guide you through the steps to cast on a top-down sock, a cuff-up mitten, cuff-up sleeve or other such object knit in the round that starts with the open end.
Cast on all your stitches onto the circular needle.
Slide all stitches to the cable and divide sts in half to set up to join in the round. Pull the cable through the mid-point of the stitches.
Slide the first 1/2 of the sts to the needle on the other end. Leave second 1/2 of the stitches on the cable with the tail and working yarn hanging. Line up the stitches to make sure they are not twisted.
Join in the round by working the first cast on stitch on the needle with the working yarn, pulling tight to close the gap.
Continue working across the stitches on needle 1.
Slide sts you just worked onto the cable and pull the needle through. This will now be your working needle.
Slide second 1/2 of the stitches to the needle. Work first few stitches a little more snug than normal to prevent a gap. Work across stitches. You have now completed a round.
Now that you have the basics, a few more pointers:
To convert a sock pattern written for DPNs, one half of your magic loop will be DPNs 1&2, the other half will be DPNs 3&4. In general, one side will be the top of the foot, the other will be the bottom.
Same will be true for mittens with one side being the top of the hand and the other will be the palm.
If you are working from a pattern written specifically for DPNs, I recommend going through the pattern and highlighting directions relating to specific needles so that you can translate to the 2 magic loop needles.