You'll probably need to add a back part first; I'd use loose extensions for the back part (a.k.a. Katie Bair method), but you could also follow a tutorial that uses wefted extensions (e.g., godsavethequeen method.)
I would've started with a shorter, thicker base wig, stubbed it, and made the pigtails out of foamcore and extensions entirely. With a long base wig, you'll need to add more fiber to cover up the bald spots, and styling it may require more patience.
In any case, I'd create the foamcore pieces (for the pigtails) first and cover them with extension fiber before attaching them to the wig. You'll probably want to wear a headband underneath the wig / build it into the wig and attach the foamcore to the headband, to make the pigtails stick out like that.