Crush the graham crackers in a food processor until they are fine crumbs. If you don't have a food processor you can use a Ziploc bag and a rolling pin to crush them.
Melt the butter in the microwave in 30-second bursts, then add it to the food processor and pulse until it looks like wet sand. If you aren't using a food processor, just mix the butter and crumbs together in a large bowl. If you squeeze the mixture together in your hands, it should hold together. If it doesn't, you may need a little more melted butter.
Press the crumb mixture into a pie dish, springform pan or baking pan, depending on what you are using it for. Use your hands or the back of a measuring cup to pack it down tightly. If you are using a pie dish you or tart pan, you can also press it up the sides of the pan.
The crust is ready to use. If you want it to be extra sturdy, you can bake it in a preheated oven at 350°F / 180°C for 10 minutes, just to solidify the crust and help it hold together. This is optional though. If you do bake the crust, let it cool completely before adding your filling.
Notes
If you aren't using your graham cracker crust right away, you can wrap it in plastic wrap or foil and store it at room temperature for 3 days, or in the fridge for 5 days.