If one forgot to separate challah from the dough, she may do so after it is baked.
If she only remembered after some of the dough was baked, she should l’chatchila bake the rest before doing hafrasha (הג' מיימוני סוף זרעים, חלת לחם בשם המר דכי ). Nevertheless, if she did hafrasha from the dough, it is fine (לקט העומר פ"ו סי"א ).
If one began eating before remembering that challah was not separated, outside Eretz Yisroel, he may continue eating and leave a piece over from which to separate challah. In Eretz Yisroel, he should immediately separate a piece from what is left.
Hafrasha is not done on Shabbos and Yom Tov (שו"ע או"ח של"ט ס"ד ) unless the dough was made on Yom Tov (שו"ע או"ח תקכ"ד ס"א ).
If one remembered on Shabbos or Yom Tov that hafrasha was not done, outside Eretz Yisroel, he should leave over some bread and separate challah from that after Shabbos or Yom Tov. In Eretz Yisroel, if there is no other choice, he may transfer ownership of the bread to a 12-year-old boy or 11-year-old girl who will separate challah with a bracha and also eat from it (לקט העומר פי"א ס"ט בשם מהרי"ל דיסקין ובספר חנוך לנער פל"ז ).
