Arg! I'm trying to remember a Willow resurrection story and it's just to the left of my memory. Willow researched the resurrection spell in secret and planned to do it "by herself." She either did do it by herself or put Dawn and Tara in some sort of compulsion spell and sucked power from them.
I do remember Willow needed a vessel to hold Buffy's body when she returned and it required a unmarked and white bull or buffalo hide. I also think the spell overwhelmed Willow and she may have accidentally left the portal open after Buffy came through and Tara had to close it.
Some of the details are a bit muddled in my mind except for the white bull/buffalo hid. | 
