Save any lengths of dough from either end. You can pinch them together to make a piece of dough large enough in which to place some dark chocolate at one end, then roll into a cylinder for a chocolate croissant. Baker's treat!
(For that matter, you can cut the 5- by 20-inch rectangle into 5 (4-inch) rectangles, place chopped chocolate or crumbled almond paste (about 1 tablespoon) at one end and roll up to make a batch of filled croissants.)
But back to the basics: Holding a triangle by its wide end, brush off any excess flour with your fingers, at the same time gently stretching the point of the dough a few inches. (This step makes the croissants flakier.)
Lay dough on the work surface and, with a slight outward motion from the slit, begin rolling up the croissant, making sure the point is tucked under so it won't unfold while baking. Place on the baking sheet, then repeat with the remaining triangles. Brush with egg wash, taking care to coat all surfaces and edges. (This step keeps the dough from drying as it rises.)
Repeat all steps with remaining half of dough. Refrigerate egg wash. (You'll use it once more.)
Let croissants rise in a cool place, uncovered, for 1 1/2 to 2 hours or until they look puffy. (They won't double in size.)