Pinkie Pie is wearing a petticoat, hence the poof.
For the long skirt, unless I'm terribly mistaken and someone can correct me, it looks like a partial circle skirt. You may be able to use newspaper to draft the pattern using the tutorials available online, and tape the pieces together. Have someone help hold the pattern around your low waist to make sure it fits correctly. Wear the proper shoes, too, to check the length. Hopefully this is correct...
Otherwise you could add a slit to a full length a-line skirt, and add a zipper along the side or back seam. Good luck!