Warm 1 ½ cups milk slightly in the microwave or a saucepan. Toss in graham crackers and stir until you have a soggy mess. Let it sit for about 10 minutes.
Pour the mixture into a fine mesh strainer, and press down on the graham cracker solids with a spatula or spoon to strain the liquid part of the mixture into a bowl {or better yet, something with a spout like a liquid measuring cup}. You should get about 1 cup of milk -- discard the solids.
Repeat if necessary {i.e. if the milk looks like it has too many little pieces floating around in it.} The milk will be slightly thickened.
Pour the remaining 1 ½ cups milk into the graham cracker milk and stir to combine.
Notes
Store the graham cracker milk in a mason jar or a container with an airtight lid. Keep refrigerated.