Options for installing linear guide rails ?

[m_c]

Ok. I am happy with that for getting the rails parallel to one another, but what about the alignment of the first rail. For example, how do most people get the rail level across it's full length making sure there are no high or low spots and ensuring the rail is linear rather than like a piece of spaghetti ?

Without having access to (very) expensive measuring equipment, a big straight edge and Mk.1 Eyeball.

A lot of people don't require a great deal of accuracy on such a large machine. For woodwork, accuracy to a few thou is good enough, and a more typical use for that size in metal is a plasma table, where accuracy to 0.25mm would be acceptable.
You need to consider what exactly you want to machine, and what kind of tolerance would be acceptable. It's all very well aiming to build a very accurate machine, but will you need the accuracy, and will it be worth the additional cost?