While building kits are available on hobby store shelves that are already in HO scale, the more you get involved in model railroading, the more inclined you may be to build a railroad structure from scratch. You can use a wide variety of construction techniques and materials, but the critical issue is scale. You need to transfer the dimension of a real structure to 1:87 scale that will look right along your HO track.
Find a measuring stick or other standard to use when shooting photos of your prototype structure. It can be a yard stick, but a longer length metal bar will be easier to use for determining scale from the photos. A 6-foot friend willing to stand up against the side of buildings when you shoot will also work. Alternately, take measurements of a doorway or other prominent feature on each side of the building.
Shoot each side of the building straight on, with your measuring standard standing straight up against a corner. Stand back far enough so you don't have to pitch your camera up to get the entire side of the structure. Don't depend on auto exposure, but instead bracket your exposure so details of the structure show up. Using a flash, even in bright daylight, will help bring out details in shadows.
Shoot close-ups of any structural features you want to capture, such as a cornice, and use the measuring tape to measure its dimensions. If you can, show the measurement in the photo.
Open the photo on your computer's photo editing program. Correct any distortion in perspective to square off the corners, and adjust the image size or printing percentage so that your measuring standard will print out at 1/87 of its real-life size. Your 6-foot friend, for instance, should print out at 0.83 inch. If your friend measures 2 inches in the image, print out at 41.5 percent. One foot in HO rounds out to 0.14 inch or 3.5mm.
Adjust the size for each photo separately and print out each side of the building for accurate plans of the structure in HO. Use the plans as templates to cut out walls and windows in wood, styrene or other building material you choose.