Big Band Arranging

I'm not quite sure what you mean. If you start with the Jazz Big Band template, it already *is* a conductor's score. Or, if you start by creating your score from scratch by selecting all the instruments yourself (not sure why you'd do that rather than use the template), again, it already *is* a conductor's score. Are you saying you currently have something else?

Do you want to assemble parts into a score?

You can select one of the templates (there's one available for "big band"), or use "choose instruments" and select all the instruments you want. It's automatically a conductor's score.