I use clear nylon thread and wrap each layer starting from the bottom up, I place each spike or bow exactly where I want it to be and wrap the thread around it three times before moving onto the next layer.
By doing this when I'm ready to attatch the clip and centre nothing is moving anywhere!!
I can also undo it if I'm not happy with the final shape
I don't glue the centre to the bow, I attached the centre ribbon to the clip, wrap it around the bow and then glue at the back.
Hope that helps
I've attatched a pic of a set of my bows.