Cut phyllo dough in half to make 9x13” sheets. You can use scissors and cut it like paper.
Make the syrup. Boil the water and sugar together for 2 minutes. Add the rosewater. Cool a bit, then refrigerate.
Make the filling. Grind the walnuts, sugar and rosewater together in a food processor until it’s almost paste-like.
Preheat the oven to 350ºF/180ºC.
Assemble the baklava. Brush the baking pan with the melted margarine. Place a sheet of phyllo in the pan, brush with margarine and continue this until you have used up half the phyllo and half the margarine.
Distribute the filling evenly over the top. Layer the rest of the phyllo and margarine, brushing the top with margarine. You should have just enough margarine; you could try using less, but it’s not something you make often, so possibly it's not worth the worry!
Cut 1 inch (2.5 cm)-wide long strips, right through to the bottom of the pan. Then cut on the diagonal to make diamond shapes. Bake for 30 minutes, then reduce the heat to 300ºF and bake about 10 minutes more, or until golden brown and puffy.
Re-cut the pieces you have already cut. Pour the syrup over the whole thing. It will look like it’s sitting in a pool of syrup, but it gets absorbed. Let it cool on a rack at room temperature for several hours before serving.