One important thing: insanity is not only (in fact, in many cases not at all) about hallucinations, it is about deranged thought patterns. Your character ought to have thought patterns and lines of reasoning that make no sense to you the writer, or anyone else. Depending on how you write, you may find it very exhausting to keep this up. I'd also suggest reading up on these phenomena in some actual psychiatric literature with examples and case studies to get an idea - I can't give you any more advice because I don't know anything myself.