Free action to declare that you will use counterspelling dice. So, you can declare how many counterspelling dice this init.
The pool refreshes at the beginning of each turn, and you use a free action to declare that you are going to be counterspelling, or you can use an interrupt action that is -5 to your init. And at that point, any spell attacks you can use any counterspelling dice you wish. So at the top of the round, you would need to use your free action again