Math congress... how does it work effectively in Grade 1?

There is currently a huge shift in education surrounding inquiry based learning. A major focus of this shift is all about Math. While I have a lot of experience with inquiry based learning through problem solving, I'm excited to make this part of my program even better next year. Last year I really focused on improving my students' abilities to talk about Math and their learning. Questions like, "What were you thinking? Show us with pictures, numbers, and words." and "How would we know what you were thinking if we couldn't ask you?" greatly helped, but I want to take their communication skills even further next year.

I was given the book Classroom Discussions: Using math talk to help students learn, and I'm looking forward to reading it.

My focus question: How do I use math congress effectively in Grade 1?