  A large set of techniques needed to compute decay rates at the two-loop level
are derived and systematized. The main emphasis of the paper is on the two
Standard Model decays H -> gamma gamma and H -> g g. The techniques, however,
have a much wider range of application: they give practical examples of general
rules for two-loop renormalization; they introduce simple recipes for handling
internal unstable particles in two-loop processes; they illustrate simple
procedures for the extraction of collinear logarithms from the amplitude. The
latter is particularly relevant to show cancellations, e.g. cancellation of
collinear divergencies. Furthermore, the paper deals with the proper treatment
of non-enhanced two-loop QCD and electroweak contributions to different
physical (pseudo-)observables, showing how they can be transformed in a way
that allows for a stable numerical integration. Numerical results for the
two-loop percentage corrections to H -> gamma gamma, g g are presented and
discussed. When applied to the process pp -> gg + X -> H + X, the results show
that the electroweak scaling factor for the cross section is between -4 % and +
6 % in the range 100 GeV < Mh < 500 GeV, without incongruent large effects
around the physical electroweak thresholds, thereby showing that only a
complete implementation of the computational scheme keeps two-loop corrections
under control.
