Unlike on Shabbat, on Yom Tob it is permissible to bake, and to prepare dough for baking. The Shulhan Aruch (Orah Haim 506) rules that one may scoop flour from the bag on Yom Tob, even though this makes a depression in the flour in the bag, which resembles digging. The prohibition of digging on Shabbat and Yom Tob applies only to earth, and thus it is entirely permissible to make a depression in a bag of flour, just as it is permissible to scoop sugar from a bag, and to take tobacco ("snuff") from its container, even though this makes a depression.
It is forbidden on Yom Tob to measure flour or anything else. Therefore, one may not collect flour in a measuring cup in order to take a precise quantity. One must make a rough estimate, rather than making a precise measurement.
It is permissible to separate Halla from dough while baking on Yom Tob, and the Beracha is recited. It should be noted that the Beracha over separating Halla is recited only when one prepares 1.56 kilograms or 3.44 lbs. of dough.
Summary: One may take flour from its bag for baking on Yom Tob, though one may not make a precise measurement. Halla is taken when baking on Yom Tob, with a Beracha.
