The ends must be parallel to within a few 1/10000 of a inch, which is not your ordinary hacksaw job, so you will need a lathe, or you might get a tilt effect.
As for distance, you typically get a set of three tubes, the length varies by manufacturer, 6, 12, 25mm or 9mm, 16mm, 28mm. etc.
The amount of magnification varies from lens to lens depending on the design. primes are the easiest to use. The tubes come in sets of three so you can vary the magnification.
Great info to have. Thanks for the response and the link.