How to Build a Mendel
Have a look at the suppliers in the Mendel Buyers Guide and follow the instructions.
You can also take a look at Mendel Build Manual which is horribly organized.
If you don't have a Mendel and are good with making stuff out of wood or steel, then here are some blueprints for the printed Mendel parts. All measurements are in mm and the Ø symbol is the diameter of a hole.