If one forgot to do bedikas chometz on Thursday night, he should do it on Friday morning with a candle and with a berachah (see 435:1). If, however, one forgot on Friday as well, he shouldn’t do bedikas chometz on Friday night with a candle, even if a non-Jew moves and carries the candle for him (even though normally we say shevus deshvus, a double derabonon in place of a mitzvah is allowed, here we don’t – see R’ Tzvi Cohen’s, Erev Pesach Shechal B’Shabbos 3:9 at length).
I saw brought down from the Shevet HaLevi and R’ Chaim Kanievsky zt”l, that even if one has a flashlight which is already on, he shouldn’t use it for bedikas chometz on Friday night.
However, the She’orim MeTzuyonim B’Halachah (115:1) writes that if a room is lit with electric lights, then one may check for chometz even on Friday night (however, it would seem that one shouldn’t make a berachah on such a bedikas chometz, as one can’t fulfil the mitzvah in its entirety, as there are rooms which don’t have lights on, or places where light doesn’t reach to). Although the Sha’ar HaTziyun (435:5) writes that one shouldn’t do bedikas chometz on Yom Tov night if he forgot to do it on erev Yom Tov, that’s because if one finds chometz he can’t burn it, and it is also forbidden to benefit from and therefore muktzeh. However, in our case, if one checks on Friday night, since the time of issur chometz is Shabbos morning, one can still benefit from the chometz, therefore, it’s not muktzeh.
On Shabbos morning one should continue checking the places he wasn’t able to check at night (i.e. places without lights). Then, if there are still more places that need checking, one should do it on motzei Yom Tov, but not on Yom Tov night (see Mishnah Berurah 435:3).
If one forgot to make a berachah on bedikas chometz, even though in a regular year one may make a berachah when burning the chometz (see Mishnah Berurah 432:4) this year, one should be concerned for the poskim who learn that the berachah is on both the burning of the chometz and the bitul [nullification] and since on Friday when burning the chometz we aren’t mevatel the chometz (as we are still going to eat it), one can’t make a berachah, and seemingly one misses out on the berachah. Unless one does like the Kaf HaChaim (432:12 quoting the Chemed Moshe) and one goes and eats chometz in a room which was already checked, and then this room is obligated once again in bedikas chometz, and then one can make a berachah.
