Before you start gathering ingredients, throw the bowl and blade of your food processor in the refrigerator to chill slightly. A totally optional step that may or may not contribute to the final product!
In the bowl of a food processor fitted with a metal blade, dump the flour, sugar, and salt. Blitz a few times to mix.
Add all the cold butter and pulse a few times, until the mixture forms little balls. Most recipes say you should get balls the size of peas. Personally, I like larger chunks of butter in a crust, so I try to stop before pea-size happens. Don't run the food processor and walk away; use short pulses and evaluate the consistency after each pulse. If you are making the crust with a pastry cutter, a fork, or your hands stop mixing when it's crumbly, and the dough is sticking into little balls. Smaller balls = a more crumbly crust and more massive balls = a more flaky crust.
Remove the blade from the food processor and dump everything into a large bowl. Sprinkle with 3 - 4 T of ice water and use your hands to bring the dough together. You want it just sticking together. If the dough turns white and gooey, you've added too much water. If it keeps cracking and breaking apart, you need to add more ice water, but add it ONE T at a time.
Turn the dough out onto a floured surface and bring it all together into a ball. Cut the dough in half, (use a scale for precision if you are that baker,) form each half into a disc, wraps the discs in plastic wrap and refrigerate for at least 30 minutes. Keeping the dough cold is what keeps the butter chunks as chunks, and chunks are what makes the crust flakey when cooked.
Now you have a dough that can be used in 30 minutes - just roll it out, fill it and bake it. Or, it can be frozen for up to 3 months. (I think the best crusts come from dough that has been frozen and thawed gently in the refrigerator), or you can blind-bake or prebake the crust after the 30-minute chilled-rest if that's what your recipe calls for.