  We present in this paper the general framework of a method which permits to
restore the rotational and particle number symmetries of wave functions
obtained in Skyrme HF+BCS calculations. This restoration is nothing but a
projection of mean-field intrinsic wave functions onto good particle number and
good angular momentum. The method allows also to mix projected wave functions.
Such a configuration mixing is discussed for sets of HF+BCS intrinsic states
generated in constrained calculations with suitable collective variables. This
procedure gives collective states which are eigenstates of the particle number
and the angular momentum operators and between which transition probabilities
are calculated. An application to 24Mg is presented, with mean-field wave
functions generated by axial quadrupole constraints. Theoretical spectra and
transition probabilities are compared to the experiment.
