I am been intending to do this for years and years and years.
Despite a total lack of interest in Classical architecture as an architectural student, I did decide in my first year of working that I really should one day draw the five orders… well here is a pathetically loose and inaccurate sketch of all 5.

These orders (often just 3- Doric, Ionic and Corinthian) are used in classical architecture. Each one has its own proportional and special decorative system. If you want to sketch classical architecture it is good to have a basic understanding of these.
Stay tuned (you might have to wait a while) for more on each order and how the orders are used in buildings and how an understanding of this helps in sketching complicated buildings.