Are you Hand Drafting or Computer Drafting? If Computer, what software? Vectorworks has some symbols in the stock libraries. For Hand Drafting, pick up a copy of this book. I would say personally, that the drafting conventions of the furniture is not as critical as the drafting conventions used for the rest of the set, as (I am assuming) that you will be buying/using pieces of pre-made furniture, rather than making, so no one is really "building" the furniture, and as long as the placement and dimensions are correct, it doesn't really matter, whereas dimensions and angles of scenic pieces to be built are more critical. UMMV.
The book referenced above, Drafting for the Theatre, is excellent and the predominant text for theatre drafting courses. If you happen to be hand drafting, there are some templates available. Do a search for "drafting template furniture". Still, those symbols are just "typical" in size and shape, so it's often best to draw your own (true in CAD also, but there will be more ready-made symbol libraries in that case, and 3D options which are nice to have). If you are working with a furniture stock that will be used again and again, or if you need to draw multiples of the same piece for a single show, it is sometimes worth making your own symbols/templates so you don't have to re-invent them later. If hand drafting, x-acto cut them out of mat board or similar.
In AutoCAD I usually use a rectangle with filleted corners and a label (couch, chair, table). The fillets seem to help the eye distinguish between the static set and the props. And drawing extra detail of the furniture adds more unnecessary lines, especially if your drawing has a lot of floor detail like carpets, traps, or multiple platform levels. Those lines can get cluttered pretty quick.