  This is the first of a series of papers devoted to a systematic study of QCD
correlation functions in a framework of 'instanton vacuum' models. The topic of
this paper is to work out approximate formulae for quark propagators in a
multi-instanton environment. As an application, and also as a necessary step
toward understanding the correlation functions, we study the propagators of
scalar and spinor quarks, using the simplest possible model, the so called
'random instanton vacuum' (RIV). Results related to heavy-light mesons, are
found to be very consistent with phenomenology.
