I’d like to learn how to fill out multiple forms (like duplicate Tax forms)

With the same data? In a Mail Merge operation? You need to provide some more info if you want any advice.

The obvious answer is, you fill the first form, finish with it, fill the second form, and so on. But I'm sure that's not the answer you were looking for. Can you give more details about what you want to achieve - and the volume of different forms?