Garam Masala means "warming spice mix" and it seems that every household has a different recipe. I lived with three Indian girls - one Malayali, one Gujarti, and one Bengali - in college and each brought their own recipes of this ubiquitous mix to our apartment kitchen. So, I just mix it up with whatever I have on hand.
Usually I use a combination of coriander seeds, peppercorns, cardamom, black cumin, cinnamon bark, crushed bay leaves, and cloves. I dry-roast those in a skillet over low heat until they are toasted and begin to let off a glorious aroma. Let cool. Then grind them to a powder in a clean coffee grinder with some ground ginger. Tonight I also added some black mustard seeds.