I think this one:-
http://www.andysshotglass.com/introduction.html
See "Collimating a Newtonian Telescope" on the right hand side.
It's a big download and the site is Flash intensive.
Collimating - how often? Depends: Type and size of scope; truss versus solid tube; whether it is moved around? transported? But generally it should be checked everytime before use. Probably secondary mirror tilt and primary mirror tilt need to be tweaked.
Also search for "barlowed-laser" for a more accurate technique for primary mirror tilt.
With the right tools and an understanding of what is happening, there is nothing to it for observing use. I suspect for astrophotography, much greater accuracy (and therefore the very best tools) is required.