5. Determine how the motherboard mounts to the chassis. Old motherboards often used several snap-in nylon stand-off spacers and only a few screws to secure the motherboard. Modern motherboards use all or mostly screws, which secure to brass stand-off spacers. The important issue is whether a given hole location in the motherboard is designed to be grounded. If it is, it will mount with a screw to a conductive brass stand-off spacer. If it isn't, it will mount using a nonconductive nylon stand-off spacer. Using a conductive brass connector where an insulating nylon connector was intended can short out and destroy the motherboard. Using a nylon connector where a brass connector was intended can cause the motherboard to operate improperly or not at all, or to radiate excessive RFI. New motherboards come with a plastic bag that contains screws and stand-off spacers of the proper type. If yours does not and you are not sure which type is required, refer to the motherboard documentation or contact technical support. If the connectors supplied with the motherboard do not include any insulating stand-offs, it's generally a safe assumption that all mounting holes are designed to be grounded to a brass stand-off connector.
6. Hold the motherboard over the chassis in the position that you will mount it. Typically all or all but one of the holes in the motherboard align with a stand-off spacer installed in the chassis. The motherboard is secured to the chassis by passing a screw through each of the motherboard screw holes and into the matching stand-off spacer. The final hole, usually the one nearest the back left corner of the motherboard, may use a slide-in spacer rather than a screw, which makes it easier to line up the motherboard with the other stand-off spacers. Most cases have many more mounting holes than are needed to secure any particular motherboard. Visually align the holes actually present in the motherboard with the chassis to determine which subset of the chassis mounting holes will actually be used. If you are building a new system, thread brass stand-off spacers into the appropriate chassis mounting holes. If you are replacing a motherboard, spacers may already be mounted in most or all of the necessary locations. Add or relocate spacers as necessary to ensure that each hole in the motherboard has a matching spacer. Don't leave any motherboard mounting holes unused. It's not that the motherboard is likely to go anywhere if you don't use all the screws. Each of those mounting holes provides support for the motherboard at a key location. If you leave one or more of the mounting holes unsupported, the motherboard may crack later when you are pressing hard to seat an expansion card, CPU, or memory module.