1 & 2 see our foundations project. The foundations need to be as shown in here.
3 & 4 Laying the blocks flat is the best way but at 5 m long, and changing direction also, you will need to put a pier in the middle. The pier should be built to double the thickness of the wall over an 18 inch length, ie an extra block laid at the back of the wall. The pier is tied to the wall either by bonding each course with the corresponding course in the wall. See our bookstore for a book on how to do this, or by laying some expanded metal lathing in the bed joint of each course. Two strips of 150mm EML required per course.