The flour for matzot is ground at least one day before it is kneaded into dough, since the grinding heats the flour slightly, increasing the risk that the dough will become ĥametz (SA 453:9).
No salt or pepper is added to the dough, since they might warm the dough, increasing the risk that it will become ĥametz (SA 455:5-6).
Le-khatĥila, one should not make matza dough with more than c. 1.5kg (c. 3.3 lbs.) of flour (the shi’ur for having to separate ĥalla from the dough with a berakha). It is hard for one person to knead such a large piece of dough thoroughly and quickly, and there is concern that parts of the dough may become ĥametz. Be-di’avad, if one kneaded a larger quantity, the matza is kosher as long as the dough did not rest for eighteen minutes and no signs of leavening appeared (SA 456:1-2).
When there are several people engaged in kneading, flattening, and rolling out the dough, some poskim maintain that it is permissible to knead larger quantities, and indeed many do so. Even so, initially it is proper to be stringent and not to knead more than the measure that the Sages fixed (MB 456:7).
When the kneading is done by machine, it is customary, even le-khatĥila, to permit kneading large volumes of dough.
One may not knead the dough in a hot place, since heat accelerates the leavening process. Therefore, one should not knead in a sunny place. Sometimes it is hot even on a cloudy day, so even if the sun is not shining, one should not knead the dough outside, nor inside next to windows, lest the heat radiate through them. Obviously one should not knead in a place warmed by an oven (SA 459:1). One authority put a number on this: one should not knead where the temperature is 30ºC or higher (Sefer Matzot Mitzva, ch. 7, n. 29). Be-di’avad, if one kneaded dough in a hot place but did not see any signs of leavening in the dough or the matza, the matza is kosher for Pesaĥ (SA 459:5).
Le-khatĥila, one may not stop working the dough for even a moment (SA 459:2). If the hands of the person kneading the dough heat up, he should cool them in cold water. Some are scrupulous to cool their hands in water occasionally while they are kneading (MB 459:27).