@meman9907 I think its similar to writing a story in general, know where you're starting, know where you're generally going, have a conclusion in mind. You don't need every session planned out in minute detail before the campaign's even begun and your plans very well could change, so plan sessions when they come. You should be familiar with all the rules but it's partly your players responsibility to be familiar with them too, so not all the pressure is on the dm.