The  analysis in R. v. W.D., 1991 CanLII 93 (SCC), 63 C.C.C. (3d) 397 (S.C.C.), which simply applies the burden of proof to any exculpatory account advanced by the defence, is well know:
·         First, if you believe the evidence of the accused, obviously you must acquit.
·         Second, if you do not believe the testimony of the accused but you are left in reasonable doubt by it, you must acquit.
·         Third, even if you are not left in doubt by the evidence of the accused, you must ask yourself whether, on the basis of the evidence which you do accept, you are convinced beyond a reasonable doubt by that evidence of the guilty of the accused.
Although there is no necessity to rigidly follow the exact steps set out R. v. W.D. it must nevertheless be clear from the reasons of a trial judge that the presumption of innocence has not been displaced by a failure to consider the totality of the evidence before arriving at a finding of guilt.
A trial judge’s instructions to a jury or reasons for judgment must leave no doubt that, in matters where credibility is in dispute, it is not sufficient to choose between one version or the other. The trial judge must consider and make it clear whether the evidence of the accused, taken in context of the evidence as a whole, raises a reasonable doubt as to guilt.
A W. (D.)instruction is not mandatory, even when an accused testifies. It was proposed as a possible approach that may assist a trier of fact to not simply treat credibility as an either/or contest. It is an explanation of the legal effect of reasonable doubt.
R. v. McCracken, 2016 ONCA 228 (CanLII).
A W. (D.)instruction might also be applicable to cases where the accused did not testify.
R. v. B.D., 2011 ONCA 51 (CanLII), 266 C.C.C. (3d) 197, at para. 105.
However, the W. (D.)decision itself and subsequent authorities make it clear that what is important is that the jury understand the burden and standard of proof. It is not necessary to slavishly follow the W. (D.) formula.
B.D., at paras. 103-104; R. v. McCracken, 2016 ONCA 228 (CanLII).

As Binnie J. said in R. v. Sheppard, 2002 SCC 26 (CanLII), [2002] 162 C.C.C. (3d) 298 (S.C.C.) the fundamental issue at the end of the trial was not credibility but reasonable doubt.