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

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

I am quite new to the whole process of creating ELT Materials however, what I have noticed so far is that students like materials which are challenging enough, fun, and eye-catching (structure, fonts, colours, instruction). Therefore, my advice would be that when creating ELT materials teachers should be careful and aware if what they have decided to include in the material is proficiency-level appropriate. Moreover, teachers are always free to play with fonts, colours, and other materials' elements thus creating something which is attractive and fun. As a consequence, students would most probably be engaged and motivated to learn even more.