I must admit that I totally forgot about this activity until I saw one of my coworkers working on it.
First of all you make a heart for every student out of construction paper. It needs to be large. Put each students' name in the middle of the heart. Pass around the hearts and have each student write something kind about that person. Each person gets their heart to take home. The students love it and are surprised at some of the comments. (Warning: Proofread each heart before passing them back. Some students will try writing mean things. )
I have done this before when I have had classes that were mean. It helps behavior for awhile. It also helps when you guilt them with a lesson first. For that, I take a heart with no name on it and crumble it up each time someone says something mean. At the end I try to put the heart back to normal. Students can see that you can't do that. It makea them think about what is said.