Yes you can prepare the meringue at least 24 hours before it is used. For preparing in advance, the process of baking will be stopped once the oven reaches 210 F degrees. This means that the meringue is stable and not prone to cracking once you store it for long periods of time. Because this temperature is maintained for an extended period, any moisture will be baked out. So you should freeze or refrigerate them immediately after cooling down and then keep them in dry containers away from any sources of heat or moisture for future use whenever needed by freezing as well as freezing in a microwave which can help remove water from egg white mixture.