I've never caught her after having her spawn, but I have a method that, for me, lets her spawn basically every time I try, including after finishing the quest (which didn't require receiving her gift, just seeing her once, extra dialogue with quest giver for having her speak with you). This works sleeping on snowed-over ground I knew to be mossy right outside my cabin, ~5 tiles from a lake, and it works in the dead of summer, miles and miles from home in the middle of a swamp. It's even made her show up at about 6pm in winter, as I was sleepy and it was dark enough for her to spawn already. She would also come back to the same spot several nights of trying to catch her in a row, so long as I didn't offend her by trying to talk. She won't trust you for a few days when you try that.
EDIT: The quest can only be failed by allowing time to expire without performing the ritual. Also you can complete the quest simply by seeing her, but there is extra dialogue with the quest giver if you don't frighten her off and allow her to speak with you. As I recall from someone on the old forum who DID get her item, it will disappear if you tell anyone about it, so if there is even more additional dialog with the quest giver when you catch her, using it might make your hard-won item disappear.
Building the fire and sleeping both have to be done on mossy ground, and the fire must be lit before midnight. After placing EXACTLY seven spruce twigs on the spot you want the fire, on top you build the rest large enough to burn well into the next day, usually an entire tree trunk and about 200 branches. Light it up and go to sleep nearby, not necessarily right next to the fire, just on mossy ground within the light. She shows up almost every time and begins her little routine of luring you into the woods. Of course, I also keep on as good of terms as I can with the spirits and usually bless the tree I cut down for the fire if I haven't done it somewhere else too recently.
Be careful in winter, I had a Forest Maiden lure me out onto soft ice and to my death by drowning. Could be the easiest way to catch her if the ice is thick enough, there are no obstructions to your vision, and you might be able to risk trying to ski after her rather than taking a torch.