Put the graham crackers in a food processor or crush by hand until completely crushed
Brown the butter in a saucepan
Remove from heat and add the brown sugar and salt
Return to low/medium heat and cook until the sugar is mostly dissolved (do not overcook - it's safer to undercook this because you don't want to burn it)
Remove from heat and add the crushed graham crackers. Stir until they are completely coated with the butter/brown sugar mixture
I like to let it cool completely and it should harden a bit. If there are any little hard bits of sugar, I run it through the food processor again. (I told you this would be annoying, but it's so worth it. Especially if you're using gluten free graham crackers that taste like cardboard.)