make sure your bowl is very clean.Dont want to sound rude but if there is any grease(even egg yolk in the mix)it will kill your meringue.
Make sure oven is preheated.
Beat to stiff peaks then fold in sweetener with metal spoon-dont use whisk or you will beat the air out of it.
Never open oven during cooking to check it.
Leave to cool in oven with door closed before removing.
Hope thats of some use and that it doesnt sound patronising.xx