Preheat your oven to 350 degrees Fahrenheit.
Divide your dough in half and begin to roll it out with a rolling pin. (See detailed hints in the post.) Your phyllo should end up being thin enough that you can almost see through it, but not so thin that it tears.
Grease the bottom of your pan with some olive oil and place your dough into your pan, being sure that it overlaps the sides. Pour the filling into your pan, on top of your phyllo. Crumble the remaining 1/3 cup of feta over the spinach filling.
Take the remaining phyllo dough and roll it out so that it is large enough to cover the top of your pan. Press down gently so that the phyllo dough sits directly on the spinach, and crimp the edges so that the top and bottom layers of phyllo are pressed together.
Cut off any excess phyllo around the edge of the pan.
Using a sharp paring knife, score the top layer of phyllo in a diagonal pattern in order to mark the serving size pieces you will cut when your spanakopita is done. Brush the top of the phyllo dough with the remaining olive oil.
Place in the middle rack of your oven and bake for 60 minutes, or until your phyllo is a nice golden brown colour.
Cool, and then cut into pieces using the scoring as a guide.Enjoy.