Instructions
Open the digital file you want to send in for printing in the photo-editing program of your choice. Most, if not all, photo-editing programs have a crop tool. If your does not, consider downloading a free program like Gimp or Paint.net, which both have crop tools. (See References)
Click on the "Crop Tool." It is usually identified by an icon that looks like four straight lines overlapping each other to form a box. Or it might simply have the word "Crop" identifying the tool.
Set the crop size to the size of the print you want, in this example 8-by-10 inches.
Place the crop tool cursor at the top left of the image. Click and hold your mouse button and draw the tool down toward the bottom right of the image until the corner of the crop tool meets the side of the image.
Release the mouse button and the crop tool will highlight the area to be cropped. Double-clicking inside the highlighted area will crop the image the way you see it. If you need to adjust the crop area you can click on the side or top line of the crop box and adjust the highlighted area. Doing this will create an 8-by-10 image, and you can prevent the top from being cropped off by doing it yourself.