Carefully unroll a roll of filo pastry sheets.
If the sheets are larger than the pan, you can cut them to the size of the pan from the start, adjust them as you go, or cut them after layering all the bottom sheets.
Place a first sheet at the bottom of the mold, then brush it with melted butter. Place a second sheet on top and brush it with butter as well.
Continue in this way, overlapping 13 sheets at the bottom, buttering every other sheet (you can use up to 15 sheets).
If the butter starts to solidify, reheat it slightly.
Note: Many chefs do not butter the sheets as they go. They directly stack the 13 bottom sheets, add the filling, cover with the other sheets, then, once the baklava is cut, they pour all the melted butter over it. I haven't tried this method yet, but I will soon, as it's simpler. This method avoids handling the filo pastry sheets too much. Spread the pistachio filling evenly over the pastry, pressing it well, then stack 13 more sheets on top (the number of sheets should be the same as the bottom layer or slightly more)
Proceed in the same way as at the bottom, overlapping the filo sheets, one buttered and one not buttered.
Butter the last sheet, then, using a well-sharpened knife or a cutter, cut off the excess pastry on the sides if you haven't done so before (see photo below)
Cut the baklava into squares, holding the sheets with a ruler to prevent them from moving.
Pour the remaining butter over the entire surface to thoroughly soak the layers.