I understand you're looking for a useful write-up or guide on how to obtain the as a PDF download.
Here is a concise, practical write-up you can use for reference or to share with others. What is it? The Explanatory Supplement to the Astronomical Almanac is the definitive reference work explaining the theory, algorithms, and data used in the Astronomical Almanac . It covers time scales (TT, TAI, UTC), coordinate systems, precession, nutation, planetary ephemerides, and observational astronomy. The most recent print edition is the 3rd edition (2013) .
