  The primordial D/H ratio now provides the best measure of the cosmological
density of baryons. We describe in detail how we deduce the D/H ratio from
absorption lines in the spectra of quasars, and we present our first two
measurements of D/H in different QSOs, which agree to within the random errors.
We describe how we correct D/H for blending with Lyman alpha forest H lines.
For our two QSOs these corrections are small, because the D lines are narrow,
deep and constrained by H and metal lines. The similarity of the two
measurements rules out ad hoc astrophysical effects, such as the destruction of
D without production of metals, or unusual data problems. We are confident that
we have measured the primordial abundance of D (log D/H $= -4.62 \pm 0.05$),
which is the first such measurement because other values in the literature are
much larger, and apparently strongly contaminated.
