Question: Any advice or tips on how to write about a world where witches exist?
I assume you mean witches with actual magic powers, not just Wiccans or herbalists.
As with any mythological being, you may find it helpful to research beliefs about witches from different parts of the world. You will find that traditions vary greatly. Witches in different places will have different types of magic, powers, natures, etc.
You have to decide which characteristics of traditional witches you are interested in and want to incorporate into your witches. Some traits you may want to exaggerate. Or you may decide to make a different type of witch that's all your own.
Then, as with any world-building, you should make a list of questions a reader might or could have about your witches. Make it a long list. Then set it aside.
Later, go back and try to come up with answers for all these questions. Don't try to make your answers definite right away. Take some time to think about them.
Ideally, you want to know a lot more about your witches than will actually appear in the book. This will make it easier to portray them in a way that feels authentic.
Best of luck.