How should I incorporate separated eggs in a cookie recipe without instructions?
I'm going to suspect you beat the whites  (all clean, no fat, no grease, no bits of yolk) to at least soft peaks, possibly stiff peaks, and fold in to the rest - cream the butter &amp; sugar (that's the beat 2 minutes step), add the yolks, add the remaining ingredients, fold in the whites. This is a fairly standard method of using beaten egg whites for a sort of "leavening" by expoiting their trapped air bubbles.

I'd be prone to putting some of the sugar and the vanilla and possibly the zest right at the end (it's got some oil so that's a little risky) in with the whites, but the way you have presented it does not seem to suggest that.