  We propose a method to exactly generate Brownian paths $x_c(t)$ that are
constrained to return to the origin at some future time $t_f$, with a given
fixed area $A_f = \int_0^{t_f}dt\, x_c(t)$ under their trajectory. We derive an
exact effective Langevin equation with an effective force that accounts for the
constraint. In addition, we develop the corresponding approach for
discrete-time random walks, with arbitrary jump distributions including L\'evy
flights, for which we obtain an effective jump distribution that encodes the
constraint. Finally, we generalise our method to other types of dynamical
constraints such as a fixed occupation time on the positive axis
$T_f=\int_0^{t_f}dt\, \Theta\left[x_c(t)\right]$ or a fixed generalised
quadratic area $\mathcal{A}_f=\int_0^{t_f}dt \,x_c^2(t)$.
