  Aims: We devise a data reduction and calibration system for producing
highly-accurate 21-cm H I spectra from the Green Bank Telescope (GBT) of the
NRAO.
  Methods: A theoretical analysis of the all-sky response of the GBT at 21 cm
is made, augmented by extensive maps of the far sidelobes. Observations of
radio sources and the Moon are made to check the resulting aperture and main
beam efficiencies.
  Results: The all-sky model made for the response of the GBT at 21 cm is used
to correct for "stray" 21-cm radiation reaching the receiver through the
sidelobes rather than the main beam. This reduces systematic errors in 21-cm
measurements by about an order of magnitude, allowing accurate 21-cm H I
spectra to be made at about 9' angular resolution with the GBT. At this
resolution the procedures discussed here allow for measurement of total
integrated Galactic H I line emission, W, with errors of 3 K km s^-1,
equivalent to errors in optically thin N_HI of 5 x 10^18 cm^-2.
