When you're starting out, it's just like any other regular 3 strand braid.

The main difference though, is that instead of preparing to bring the braid towards yourself to continue it, you should be preparing to keep the braid raised up enough so that you can reach under it to add hair to the middle strand.

See?  You're gettng ready by orienting the braid so that you've taken the strand on the left, and are moving it to the middle with your right hand.  By the same, you will take the strand on the right, and move it to the middle with your left hand, but you're going to do something else while you're at it...

Now, while you're taking the strand from the right, and moving it to the middle, you're going to pick up some hair from further along the section.  You must join the added hair with that hair that you're moving toward the middle in order to make it one united strand.

From here, it's just a matter of a cycle...Reach under and pull up, reach under and pull up, reach under, and so on...

Hand positioning is crucial to a good turnout.  Different methods and positions work for different people but remember that your objective is to get the hair that will be added to the middle strand (or rather the strand that is going to be in the middle) from under your other hand.

Some add hair to the outside strands, but be warned that for many people, this is confusing. It resembles French braiding too closely, and those who are used to French braiding may get mixed up.

Video:

This video demo should help with hand positioning.  The quality is home made, and I apologize, but it will help you get the idea.