How do I know where to file? What do you use to file/trim?
A pedi-paws, like ramirezframing said. I have a shop dremel I use on the lowest settings and it does the same thing. Just a more powerful tool. I have had success with large, heavy grit nail files, also. I have been able to find them anywhere from the dollar store to Walmart. Just have to go to the nail and beauty and look for them, typically in a bucket or a flat package on the racks. Just go slow and shape it. You can see some parts of the beak that are kind of see-through-ish. It's hard to explain, but it's easier once you really get working on them. You will see a line of growth and then a more solid end where it forms the quick. Kind of like a nail where you have the growth and then the part that attaches to the finger. Just gently trim away with the file or dremel until you get that excess growth off. If you are using a dremel, you often will get a tiny drop of blood if you are still learning where the quick is. If you nick them, just shape and round it down but don't take it back any further.
You should be able to see on the sides of the beak where it has a bit more growth. It's like shaping a lid for a tea kettle! You just need to gently shave off what you can so that the beak seats correctly. My crossbeak's face wont seat correctly. I take down the sides as best as I can so it has a more solid fit. I am hoping, eventually, if I keep at it that the beak will either stay the same so he can still eat/drink OR get a bit better with grow out. We'll see. If I can, I may try to do a picture guide! Just hard to do that and hold a chicken at the same time, lol!