Make sure you turn off the noise layer. Then flatten the file. Then crop.
And yes, save it as a new file.
https://damiensymonds.net/2010/02/trash-those-jpegs.html
THEN add your noise.
Just choose your crop tool and enter the size and resolution.
(Check with your lab to see what resolution they require. Most say 300ppi, but some differ.)
There are two things wrong with the file (I only opened the first one of the batch you sent me).
You've added way too much noise to the background.
You didn't resize and sharpen accurately for print.
What you're doing wrong is trying to edit photos before you've learned to do so!
You've taken the Layers and Masks Class, which is NOT an editing class. It just teaches you the tools.
The Levels Class teaches you how to edit.