Which ONE piece of advice would you give to a teacher who wanted to create their own classroom materials?

Which ONE piece of advice would you give to a teacher who wanted to create their own classroom materials?

I think the most important tip would be to always start with the end in mind. A teacher must always know the purpose and the objective of the lesson or the classroom material they wish to create. Is it to improve students' ability to speak fluently? Is it to improve their writing? Keeping your classroom material focused would help keep your class organized.