Until you’re ready to layer the phyllo sheets in the baking dish, keep them covered with plastic wrap topped with a damp towel. This keeps the thin sheets from drying out.
Brush the edges of the phyllo with butter first, then work toward the center. And don’t forget to brush the top layer for a golden brown crust.
You can substitute 2 lb fresh spinach for the frozen. Remove the stems and toss in oil just until wilted. Place spinach in a colander to squeeze out excess liquid, then roughly chop it.