Thaw store-bought puff pastry slowly in the refrigerator, in its original package. Thawing it at room temperature or in a microwave melts the fat and destroys the layers before they can rise. Work with the pastry cold and do not roll it too thin. Always cut the edges with a sharp knife in one stroke; crushed edges will not puff in the oven.
Thaw the pastry in the refrigerator and leave it in its package. Depending on its thickness, this takes a few hours and often overnight. At room temperature, the fat in the outer layers melts, runs into the dough, and damages the layers before it is rolled out. Never use a microwave. If you are in a hurry, do not submerge the package in cold water; move it only to a warmer shelf in the refrigerator. Do not open the package to wipe away moisture on top while it thaws. Open it only when the pastry is fully thawed and your work surface is ready.
Work quickly while the pastry is cold. Dust the counter with a little flour, roll out the pastry, and do not roll it thinner than needed; store-bought puff pastry is already made to the intended thickness, and rolling it too thin crushes the layers. Turn the dough as you work rather than forcing the rolling pin in one direction. If the pastry starts to soften, stop and put it in the refrigerator on its tray to rest for 10 minutes. Keep the work surface cool as well; on a warm counter, the pastry can soften and start sticking to the rolling pin within a few minutes.
Cutting is where many mistakes happen. Use a sharp knife or pastry cutter, and cut straight down in one stroke. Sawing back and forth or pressing with a dull tool sticks the edge layers together, and those edges will not rise in the oven. For shapes cut with a mold, press straight down without twisting the mold. Do not brush egg over the cut edges; it glues the layers together. Working directly on parchment paper instead of a cutting board is also common; it prevents the pieces from being damaged or their edges crushed when you move them.
Puff pastry needs high heat because steam released as the fat melts is what separates its layers, and that steam needs to form quickly. In a lukewarm oven, the fat melts slowly, runs into the dough, and the pastry does not rise. Line the baking sheet with parchment paper. Put the pastry into the oven while it is cold; if you need to wait after preparing the tray, keep it in the refrigerator, not on the counter. Prick areas that should not rise with a fork. For tarts that need a flat base, skipping this step can make the center bulge and the filling spill over.
Do not refreeze thawed puff pastry; doing so damages its texture again and keeps the layers from opening. Thawed pastry keeps only briefly in the refrigerator and should be used the same day. Do not stack leftover pieces, because the cut surfaces can stick together; if you need to stack them, put parchment paper between them. Do not throw away scraps, but do not knead them again; stack them, press lightly, and roll them out again. Check the best-by date and look for ice crystals in the package. Visible crystals indicate that the pastry thawed and refroze once during transport; do not use it.