The present invention relates to a method for forming packages of flexible material with a first side gusset and a second side gusset starting from a strip coming from a reel having a sliding direction. The method comprises the following steps: making the first gusset by folding close to a first side edge of the strip coming from the reel, and making the second gusset by folding the strip coming from the reel; in which the step of making the second gusset is performed on a portion of the strip which extends in the sliding direction after the step of making the first gusset was performed on the portion of the strip. Moreover, the present invention relates to a machine configured so as to carry out the method.