I did find this one site that provides an example of how to do it.. http://www.livinghome.com/news/gardens/178-1.html
However it does not answer my other questions.
Do I need to use a special type of mortar?
If I would like it to be higher than say 18" do I need to provide some type of ventilation
any other suggestions or comments greatly appreciated also feel free to email me if you prefer. thank you,