Aside from nuts and screw, you may also want washers to spread pressure over a greater surface area to reduce the likelihood of destroying things and plastic/rubber bushings/washers thick enough to conform around any SMD components that might be on the board's bottom to put between the steel washer and motherboard to prevent short-circuits and crushed SMDs.
Since rigid mounting like this will ratchet pressure up quite fast, bring all screws to evenly snug to make sure the block is level on the IHS, then give the screws about an extra turn by giving them an extra half-turn in an X-pattern.
Push-pins are the only way to install a heatsink without taking the motherboard out when the motherboard does not have some other compatible mounting mechanism pre-installed. Many cases have a motherboard tray cut-out to allow access to the CPU area by removing the side-panel, so you may want to check for that first if you don't already know al whether this is the case in your case.