This hairstyle turned out really cute and I wish these pictures did it better justice but we were in a condo on vacation and these were the best shots we could get. 
Make sure you start with wet hair and create a high part going from one ear to the other so that you have a "headband" like section in the front. Now you are going to create 8 small sections out of that front section you just created. (4 on each side.) Secure each of those 8 sections with a small elastic. 
Now you've got 8 little ponytails that you will pull out of the way and have your princess hold for a minute. Divide the remaining hair in half by creating a part directly down the center of the head. Pull each side into a low ponytail.
Now bring those 4 ponytails from the left side of the head and add them into the ponytail on the right side with a new elastic.
You will now weave the 4 small ponytails from the right side through the left side ponytails using an alternating over and under method. (I used a Topsy Tail to make it a bit easier.) Once they have been weaved through you can secure them into the ponytail on the left side with a new elastic.
I've included this picture to show you what my weaving looked like in the back once I finished. You can see that they are not crossing directly over that center part like they should so I just adjusted them with my fingers a bit until they all cross over that center part and then sprayed them with hairspray to make sure they stayed put.
You can leave your two ponytails loose or finish them off however you like. I've done a 5-Strand Braid out of each and you can find the instructional video for how to do that here.