Answer:
To defeat Yin Yarn you have to defeat the second Fangora he makes first, then the second Campari. The rest I don't know the order, but here's how to defeat him.
Needles and a Curtain: Yin Yarn will drop a curtain and these weird sewing needles will appear. They make seams you jump on! First they will try to attack you by making giant X's (the needles hurt Kirby! Dodge them!). Then they start sewing little lines you can jump onto. There's a door at the top. Get to the top and press UP on the Wii Remote. (You have to be quick, the lines disappear) Yin Yarn will use this attack several times.
Hocus ENEMY: Yin Yarn will create several enemies, like Clone Waddle Dee's and Clone Scarfy and Button Wasps and Clone Waddle Doo's. When he makes Scarfy's (the cute flying teddy heads that turn evil when they spot you) get hit by them or they won't leave.
When he makes Waddle Dee's or Waddle Doo's, grab them and make a small yarn ball out of one. Throw it at Yin Yarn. BAM! He gets hurt. Quickly grab his button and pull!
The Ultimate Yin Yarn!: When you defeat Yin Yarn and you're dancing - you haven't defeated him! His knitting needles will talk to each other, then use the Magic String to make ULTIMATE YIN YARN!!!! When it 'seams' like the end, Meta Knight will suddenly drop a portal thing, used for Kirby to TRANSFORM! You will become Robot Kirby and battle Yin Yarn. If you're One Player, just keep pressing 2 on the Wii Remote. If you have Two Players, use your own battle strategy! Yin Yarn will explode.
Mission Complete: The knitting needles will run away. You follow them (no matter what) and turn normal again. You and Prince Fluff will trap them (Two player or One player) and grab them. Then you both hurl them and BYE BYE KNITTING NEEDLES. MISSION COMPLETED!