The oven should be heated thoroughly, so that the dough begins to bake immediately. If the heat is low, the dough might begin to become ḥametz before it bakes. Clearly one may not bake matza in the heat of the sun, and if he did so, then even if the heat was very strong and it is obvious that the dough did not leaven, one does not fulfill the mitzva of eating matza with it, as the Torah calls matza the “bread of affliction,” and something sun-baked is not properly called bread (SAH 461:6).
It is not necessary, however, to bake the bread specifically in the flames of the fire. Rather, even if filaments burn under a metal or earthenware plate, as long as the plate is burning hot, one may bake on it (SA 461:2). Similarly, one may bake in an electric oven whose heating elements glow hot, for that is considered like fire. However, there is uncertainty regarding matza baked in a microwave oven, as some maintain that it is unfit for the mitzva of eating matza since it was not baked by fire. (R. Shlomo Zalman Auerbach disqualified such matzot, and R. Shaul Yisraeli held them to be kosher; see R. Harari’s Mikra’ei Kodesh, p. 335.)
One may not decorate matza with pictures, lest it become ḥametz during the delay caused by decorating. One may not make a thick matza (thicker than 7.6 cm) on Pesaḥ, out of concern that the fire will not reach the center and it will become ḥametz (SA 460:4-5). However, one may bake a matza that is a bit thinner than a tefaḥ. Ashkenazic custom is to make thin, hard matzot, as this way the fire reaches them thoroughly and there is hardly any concern that the matza will contain ḥametz (Rema 460:4). Some Sephardim bake matza that is about as thick as a finger, while others make them thin like wafers, as Ashkenazim do, since they usually bake them well before Pesaḥ, and if they are not wafer-like, they will not last long (Kaf Ha-ḥayim 460:44).
One does not fulfill his obligation with a stolen matza (SA 454:4). Sometimes a purchaser takes the matzot into his possession without paying immediately. If the seller indicates that he wants to receive payment immediately, the purchaser must be careful to pay as the seller requested. If the seller seeks the buyer out, demanding payment for the matzot, and the purchaser dismisses him by saying “come back later,” then the purchaser does not fulfill his obligation with those matzot, because they do not belong to him (MB 454:15).