Sketch, photo or video. Formal drawings or a model are not necessary, as long as the searcher can understand the structure and operation of the invention.
Brief description. Your description should direct the searcher's attention to the features you think may be patentable (or infringing). Describe what the invention is, not just what it does. The advantages of the invention are not as important as its structure (or its steps, if the invention is a method).
Be concise. A long disclosure can hide the most important elements and may consume time that could otherwise be spent searching.
Identify "prior art". List relevant patents, publications, web sites, products, manufacturers and inventors you know of.
You may download an invention record form here.