Roll out the bottom crust on a lightly floured surface and transfer it to a 9-inch pie plate. It should overhang slightly by about ½-inch or so. Do not trim of finish the crust edges at this point. Fill with your pie filling, which should be cooled completely if previously cooked (like some apple, blueberry, or cherry pie fillings).
Roll out the top crust on a lightly floured surface until it is about 3-inches larger in diameter than your pie plate (so about 12-inches in diameter). Use a pastry cutter or pizza cutter to slice strips of pie dough for the crust. They can be a uniform width or you can vary the width for visual interest or a more unique pie design. I tend to stick to a fairly traditional ¾ to 1 ¼-inch wide strip for my lattice crust as it is easier to work with than narrower strips. A ruler certainly helps maintain straight lines and even strips, or you can just eyeball it if you have a steady hand.
Lay 4 to 7 strips (depending on how narrow or wide your strips are) going one direction across your pie (I prefer vertical, but it doesn't matter), spacing them slightly apart. Use longer strips for the middle of the pie and shorter strips as you work your way toward the edges. Fold back every other strip halfway.
Add your longest strip of unused pie dough perpendicularly across the other parallel strips. Return the folded strips back to their original position. Fold back the alternate parallel strips over the perpendicular strip.
Add another perpendicular strip across the initial parallel strips of dough, then return the folded strips back to their position. Continue on with this pattern until you have used up all of your pastry or covered the top of the pie with a woven lattice crust.
Trim the lattice strips around the edges, then fold the bottom crust over the ends of the strips and crimp around the circumference of the pie for a finished look. Alternately, you could fold the lattice strips and bottom crust under for a different look and skip crimping.
Refrigerate for 30 minutes before baking. Pie crust bakes best when it is cold and the handling and time it takes to weave a lattice crust usually allows the dough to warm up enough that you will want to refrigerate the unbaked pie for a bit to make sure that the lattice design holds its shape while baking.
Brush the top of the pie crust with an egg wash made from 1 egg beaten with 1 Tablespoon of water or milk so it will bake up golden brown. Bake according to directions listed in the recipe you are using.