From working in an espresso bar:
Whatever syrups you can get will work (we had Da Vinci, Monin is probably better). The amount is totally subjective, start with a shot (1oz.) and go plus or minus depending on your wife's taste.
Pour flavoring into cup
Pour espresso into cup
Pour milk over the top
It mixes pretty well on its own, but if you have a really thick syrup (chocolate is the only thing I can think of), you may want to draw the shot, then pour in the espresso and a little milk and stir to dissolve it and then top it off with the rest of the milk.
Try adding it to the milk and frothing, however, may turn out unique and good.
With that stuff, it is all about experimentation to meet her tastes.