  We put forward a method to tune the polarisation state of decaying heavy
particles (top quarks and $W/Z$ bosons) in a pre-existing Monte Carlo sample.
With this technique, dubbed as `custom angle replacement', the decay angular
distributions are modified in such a way that the desired polarisation state is
reproduced, while all the production kinematics are unchanged. A non-trivial
test of this approach is presented for the top quark semileptonic decay $t \to
W b \to \ell \nu b$, with $\ell = e,\mu$, in which the decay distribution is
four-dimensional and involves three different Lorentz frames. The proposed
method is ideally suited to obtain event samples with polarised heavy
particles, as required by experimental measurements of polarisation and spin
correlations.
