  We propose a powerful and convenient method to systematically design
flat-band lattice models, which overcomes the difficulties underlying the
previous method. Especially, our method requires no elaborate calculations,
applies to arbitrary spatial dimensions, and guarantees to result in a
completely flat ground band. We use this method to generate several classes of
lattice models, including models with both short- and long-range hoppings, both
topologically trivial and non-trivial flat bands. Some of these models were
previously known. Our method, however, provides crucial new insights. For
example, we have reproduced and generalized the Kapit-Mueller model [Kapit and
Mueller, Phys. Rev. Lett. \textbf{105}, 215303 (2010)] and demonstrated a
universal scaling rule between the flat band degeneracy and the magnetic flux
that was not noticed in previous studies. We show that the flat band of this
model results from the (over-)completeness properties of coherent states.
