Well, first and foremost, you're going to have to be less concerned about how others will think of you. The worst thing that could happen is people will see you opening up and being more social. Oh, God forbid!
DO butt in... but only if you have something to add to the conversation. Don't butt in and be all like, "Haha" and then leave it as such. Remember, your goal is to let people know that you're someone they can interact with, not to have them look at you with this kind of face: o_O
If there was a new student, tell that person, "Hey, come sit over here. This is the fun side of class!"
Can't give you any specific steps because I don't know them. And if I did, I wouldn't tell you. Thing is, you can't live your life like a textbook example. At times you will fail and you will be embarrassed, but other times you will succeed and you'll be more happier when you do because you allowed things to happen naturally instead of going by procedures. Take it easy man, you can do it.