First thing you want to do is create a detailed sketch of how the illusion will be created over the space.
Then the next step is to draw out the perspective dimension over the area. This can take from 8hrs to a couple of days..but this must not be of concern, becasue doing it right will leave a lasting impression.
3d illusion complete..this one took a few days and can have much more detail depending on desired result. See more
http://3dillusionart.com Happy painting,if this tutorial helped you please leave feedback thanks!