First, prepare the filling. Chop nuts to desired size. Melt butter in the microwave, and add sugar and cinnamon. Mix to combine. Add the nuts and stir, let stir for a few minutes to let the butter cool down a bit and the nuts to absorb some moisture.
Lay a phyllo layer, and brush it with melted butter. Lay another layer, and brush it with more butter. Lay a final layer, brush it with butter, and add 1/4 cup of the nut mixture at the bottom of the phyllo horizontally. Roll the phyllo layers into a horizontal tube, making sure the nuts stay inside. Try to make the roll tight, without taring the pastry.
Continue working this way until the baking sheet is full / there are no more nuts left. With a sharp knife, cut through the baklava rows vertically. Brush generously with butter.
Bake until golden. Make sugar syrup by bringing all of the ingredients to a boil. When the baklava is ready, pour the syrup over it.