When your original file size is not the same aspect ratio as an 8×10, the image will distort if you use the image size in Photoshop with your constrain proportions unchecked to size the file. You can crop your file one of two ways to be an 8×10 size: (1) use the cropping tool in Photoshop to size the file to 8×10 at 300 ppi. However, you will lose some of your image. Or (2), you can use the cropping tool but only set the dimensions to 10". Crop your file, then go to Image > Canvas Size in the menu bar, and enter a canvas size of 8×10. You will have a file sized at 8×10 without cropping or losing any image, but with white borders on 2 sides. You can also use this recanvasing method for printing a custom-sized print with us.
If you are cropping your files in Lightroom this is done from the Develop screen by setting your file dimensions and then dragging the crop tool over your image.