When to Do Hafrasha

Hafrasha should not be done from flour. If one did, it is not called challah (שו"ע סי' שכ"ז ס"א ).

L’chatchila, hafrasha should be done in the dough stage (ש"ך ), immediately after kneading, so as not to forget. If one did not do hafrasha before baking the dough, one may do it afterward (שו"ע שכ"ז ס"ה ).