  By analyzing the external field dependence of correlation functions, the
magnetic properties of hadrons can be determined using lattice QCD in magnetic
fields. To compute the magnetic moments and polarizabilities of charged
hadrons, for example, one requires sufficiently weak magnetic fields. Such
field strengths, however, lead to closely spaced Landau levels that are not
straightforwardly resolved using standard lattice spectroscopy. Focusing on
charged spinless hadrons, we introduce a simple projection technique that can
be used to isolate the lowest Landau level. As the technique requires the
explicit coordinate-space wave-function, we investigate the extent to which the
continuum, infinite volume wave-function can be employed. We find that, in
practice, the effects of discretization can be handled using a perturbative
expansion about the continuum. Finite volume corrections are taken into account
by using the discrete magnetic translational invariance of the torus. We show
that quantized magnetic fields can lead to pernicious volume effects which
depend on the magnetic flux quantum, rather than on the lattice volume.
