Hi! Welcome to DD
Here are the steps I take:
- Open the photo
- Select File > Duplicate (In Photoshop Image > Duplicate) to create a copy of the photo so that you aren't working on the original.
- Get the Crop Tool.
- In the Options Bar across the top of the screen, type your desired height, width and resolution. Type 300 ppi for print quality.
- Click and drag out a square over your photo.
- Click the green checkmark to confirm the crop.