From about 50 to about 90% by weight of fibers, in particular wood fibers, and/or pellets made from cellulose or from wood, are mixed with from about 10 to about 50% by weight of a hot-curing resin, each of the percentages by weight being based on the total weight of the mixture. Instead of a single resin it is also possible to use a mixture made from two or more hot-curing resins. The mixture made from fibers and/or pellets and resin is kneaded and homogenized and then passed on to an extruder which has various heating zones in which the mixture is heated. Care has to be taken here that the maximum temperature, arising at the extruder exit, does not exceed about 120° C., since above this temperature the resin(s) begin(s) to cure. The mixture, not entirely cured, is extruded as a homogeneous composition and introduced into a calender line in which pressure is exerted on the composition to an extent which gives a preform mat, the shape of which is stabilized, and which is then removed for further processing or storage.