Depends on the wig what kind of issues you'll have pulling a loose wig into a ponytail. I'd start just gently finger combing it back to see where you would even have holes or an issue with the net showing through. Most wigs are meant to be worn in the style you buy them as so making a full ponytail is gonna be hard.
Easiest way is to fill any gaps you're gonna have before hand by I guess sewing small wefts in the new direction? Then starting at the very back where the ponytail starts gather the hair layer by layer, kind of like in larger rings going away from where the ponytail is going to be centered.
You might have to glue that in place and buy a clipon ponytail just to get it to sit right. It all depends on the wig though where the holes are going to show if any.