Divide the dough into four pieces. Work with one at a time - flatten the piece with your hand or a rolling pin before passing it through the thickest setting on your pasta machine. Fold the edges in to create a neat rectangle that fits the width of your pasta machine, and run it through the thickest setting again until your dough is uniform in shape.
Continue passing your dough through the machine, working through until you get to setting 4 or 5 (on a Marcato machine) - we want the dough relatively thick for this so that the garganelli will keep its shape when rolled against the pettine.
Cut the sheet of dough into either 3cm or 4cm squares with a straight wheel cutter or knife.
Place one square at a time diagonally on the pettine or gnocchi board so you have a corner at both north and south.
Place a small dowel (or the handle of a wooden spoon) horizontally across the middle of the piece of dough.
Roll the bottom corner over the dowel to meet the other side, then continue rolling the pasta away from you to connect the top edge, forming a tube (with a ridge pattern on the exterior).
Gently slide the garganelli off the dowel and set aside on a baking sheet lined with fine semolina.