Place one phyllo sheet in the pan and brush it with melted butter. Place the second phyllo over it and butter it too. Repeat this until you have 5 sheets in total.
Repeat with another 5 phyllo sheets and walnuts on the 5th.
Repeat with another 5 phyllo sheets and walnuts on the 5th.
In total, you will have 25 phyllo sheets and 4 layers of walnuts (between each 5 phyllo sheets).
Remove it from oven and using a ladle, pour cooled syrup over hot baklava. Let it sit uncovered for 4-5 hours so that the syrup spreads well into the layers.