I'm a tile/stone mason,and if I was to build my own.....I would start with a general plan as far as rough in,and finish dimensions.
Once you have the plans and permits,you can then proceed with your project. You need to consider water table,area for your spa,electrical,plumbing,and service pit/area for your equipment.
As I play in the mud on a daily basis,for me,it wouldn't be too hard. I would consider the different aspects of your project,prior to committing to it. Also,as far as I'm aware,a concrete/mud hot-tub doesn't have the hydro therapy(jets/attachments) that a portable spa can provide.
In either event,GL
