Before you start modeling: What are you going to make?
It's important to have some kind of blueprint before you jump right into making the model. I recommend drawing out how you want your plush to look on paper. That way, you can make quick adjustments without having to remake parts of your model.
At most, draw how you want your plush from the front. However, I also recommend doing a quick sketch for the back and side views as well. If you really want to get fancy, you can draw out sketches for individual pieces to reference as you model.
The drawings don't need to be fancy! As long as you understand your vision, that's all that matters.
Once you are happy with your concept, it's time to start on your model!