The melakha of Meraked (sifting) prohibits sifting flour. After flour has been ground up, coarser particles of bran from the outer layer of the wheat kernel remain. To separate the flour from the bran, it is sifted with a sieve. The fine flour filters through, while the coarse bran remains on the surface. As is the case with all melakhot that are generally done for the long term, this melakha is forbidden on Yom Tov.
However, if one wishes to re-sift flour that has already been sifted, whether to improve it before kneading it or to remove a foreign object that fell into it, there is no prohibition. Nevertheless, in order to make it clear that this sifting is being done for same-day usage, the Sages required a slight shinui. For example, if one generally sifts into a bowl, he may sift onto the table or with the sifter upside-down (Beitza 29b; SA 506:2). This rule applies to the packaged, pre-sifted flour that we buy nowadays: one may re-sift it on Yom Tov with a slight shinui to improve it or check it for bugs.
It is permitted to perform Lash for food that will be eaten on that day of Yom Tov. Thus, one may knead dough to make bread or cake or prepare instant mashed potatoes. However, one may not knead in order to prepare food for the weekday, for non-Jews, or for animals. Of course, it is forbidden to mix water and dried earth in order to make bricks. (See Peninei Halakha: Shabbat 12:3-7 for the details of the melakha of Lash.)
If one produces enough dough for the mitzva of hafrashat ḥalla to apply, the ḥalla must be set aside after kneading. Though the Sages prohibited setting aside ma’aser and ḥalla on Shabbat and Yom Tov, this is because doing so looks like one is fixing or improving the food, since the bread or produce may not be eaten until these parts have been removed. However, if the dough was not made until Yom Tov, ḥalla may be set aside from it since the obligation of ḥalla only came about on Yom Tov, and it was impossible to do the mitzva beforehand. Since baked goods may not be eaten until ḥalla has been set aside, the permissibility of preparing them for a Yom Tov meal implies permission to separate ḥalla on Yom Tov (Beitza 37a; SA 506:3).
Back when Kohanim were ritually pure, the designated ḥalla would be brought to them on Yom Tov so that they could enjoy it. However, now that Kohanim are impure and may not eat ḥalla, during the week many people who separate ḥalla burn it in order to make sure that nobody accidentally eats it. However, doing so on Yom Tov is forbidden, since it serves no Yom Tov purpose. Rather, it is proper to wrap the ḥalla in paper and put it in the garbage. If the garbage can contains filthy items, the ḥalla should be double-wrapped, rendering the disposal more respectful. Once the ḥalla has been separated, it is muktzeh, but as long as it is still in the hand of the person who set it aside, he may wrap it up and take it to wherever he plans to dispose of it (SA 506:4; MB ad loc. 29).
. Working with our updated understanding of shi’urim, one who bakes bread using 1.5 kg of flour or more should set aside ḥalla with a berakha, while one using between 1.1 and 1.5 kg sets aside ḥalla without a berakha. According to R. Naeh, one recites a berakha on 1.666 kg or more, while setting aside ḥalla without a berakha is appropriate when using from 1.25 to 1.666 kg; this larger quantity is based on R. Naeh’s calculation, which is based on the Turkish dirhem (drachma), which is more than ten percent larger than the dirhem of the Rambam (see Peninei Halakha: Berakhot ch. 10 n. 11).