When you fill the filter of your Moka pot, make sure to level off the coffee grounds - do not tamp down as you might do in an espresso machine. This is because Moka pots work with the pressure building up from the steam, and packing the grounds might restrict the water flow, leading to a burnt taste. The quantity usually depends on the size of your Moka pot, but generally, filling the filter funnel to the top with coarse grind coffee works best.