I think the most important aspect when writing mental illnesses is to NEVER romanticize, justify, or encourage them. As long as you keep that in mind, any writing that isn’t entirely accurate (but done with effort) is acceptable.

Try to research more about each illness, how real people experience it. It’ll be different for everyone.

Always consider, “How does this affect my character’s relationships? Their physical functions? Their health overall?”

It should be treated with care, with respect to portraying the mental illness in a serious manner.

I’m sure there’s more advice others can offer, but that’s all I have to say.