Simple. Someone names an anime character (be sure to include the anime they come from), and the next person names another character to pair with that character for a shipping pairing (no limitations. You can even pair characters from separate shows. You might want to watch who you pair with who, though). Here's a few examples.

Poster #1: Light Yagami (Death Note) would be perfect for...
Poster #2: Misa (Death Note). Boa Hancock (One Piece) would be perfect for...
Poster #3: Kenshiro (Fist of the North Star). Rangiku Matsumoto (Bleach) would be perfect for...
Poster #4: Kiyone Kotetsu (Bleach). Cyclonus (Transformers Armada) would be perfect for...
Poster #5: Bulkhead (Transformers Energon). Zoey Hanson (Mew Mew Power) would be perfect for...