Separating the piece. Some pull off a piece, make the bracha holding the piece, and then state, “הרי זו חלה ” (רמב"ן סוף הל' חלה ).
Others first make the bracha and then pull off a piece. In that case, it is clear that the piece is challah, and it is unnecessary to say “הרי זו חלה ” (בן איש חי ש"ב פ' שמיני ). Still, some say it even in this manner.
One should have in mind that only the dough which readily comes off her hand will be challah, not any dough that sticks to her hand. This avoids possibly getting the challah portion mixed with the rest of the dough.
When doing hafrasha from dough containing between 1.2 and 2.25 kg of flour (above, 9-10), one should have in mind to cover flour added while shaping the dough, as a bit of extra flour can cause a full chiyuv.
Nusach of the bracha. Sefardim say “...וצונו להפריש תרומה ” (שו"ע שכ"ח ס"א ), while Ashkenazim say “...להפריש חלה ” The main Ashkenazi minhag is to say “להפריש חלה מן העיסה ” (ט"ז ). If hafrasha is done after baking, one should just say “להפריש חלה ” (לקט העומר פ"י הערה י"ט ).
Even if one merely had in mind to designate a piece as challah without saying “הרי זו חלה ” or making the bracha, the piece becomes challah (תורת הארץ אות ל"ג ).
It is a good idea to place the challah portion in a plastic bag immediately after separating it. This makes it clear that the piece is challah and prevents it from getting mixed up with the rest of the dough, an otherwise common occurrence, which leads to shailos (below, 54).
