Once your portal updates with the actual application, you will find the instructions about taking the CFA. You can take whenever you want, but make sure you're ready because I believe some regions do not admit resending CFA scores. So do your best, and treat averages as minimums (and max as average... hahaha).
It depends on your nomination pack whether they ask for the CFA scores or not. They didn't ask it for me.
You'll also (officially) take it during SLS, if you attend it, but you will be able to resubmit it if you wish (so if you did badly, don't worry). Might wanna double-check this with your RC.
Good luck!