  We present an analysis of the integrated properties of the stellar
populations in the Universidad Complutense de Madrid Survey of Halpha-selected
galaxies. In this paper, the first of a series, we describe in detail the
techniques developed to model star-forming galaxies using a mixture of stellar
populations, and taking into account the observational uncertainties. We assume
a recent burst of star formation superimposed on a more evolved population. The
effects of the nebular continuum, line emission and dust attenuation are taken
into account. We also test different model assumptions including the choice of
specific evolutionary synthesis model, initial mass function, star formation
scenario and the treatment of dust extinction. Quantitative tests are applied
to determine how well these models fit our multi-wavelength observations for
the UCM sample. Our observations span the optical and near infrared, including
both photometric and spectroscopic data. Our results indicate that extinction
plays a key role in this kind of studies, revealing that low- and
high-extinction objects may require very different extinction laws and must be
treated differently. We also demonstrate that the UCM Survey galaxies are best
described by a short burst of star formation occurring within a quiescent
galaxy, rather than by continuous star formation. A detailed discussion on the
inferred parameters, such as the age, burst strength, metallicity, star
formation rate, extinction and total stellar mass for individual objects, is
presented in paper II of this series.
