Garam masala is the most important spices mix in any Indian kitchen. It literally means 'hot spices'. The word 'hot' here means spiciness as opposed to temperature hot. Garam masala is common to India, Bangladesh and Pakistan. There is no one recipe to make this mix. Keeping the basic coriander seeds, dried red chillies, cumin seeds, cloves, cardamoms and cinnamon sticks, you can add any of the other spices or condiments to get the mix you want. Ideally the same mix is not used for all dishes.
Of course there is personal preference which would make you change the proportions of the ingredients. Where a thick gravy or base is required, more coriander seeds are taken. These add the body keeping their flavor as delicate as ever. Garam masala also differs from region to region in the same country. Mixing your own garam masala calls for creativity. There are dishes where you might want to dry roast all the ingredients before grinding them to a powder.
Another method is to roast the masalas with a little oil and then grind. Of course there is the commonly ground masala where you grind the ingredients without roasting. Nutmeg, mace and star anise when ground with the usual masalas, add a distinct flavor. There are some dishes in which ground garam masala is put into a muslin bag and placed in the vessel in which meat or even rice is cooking. The flavours enter the dish by infusion.
Mughlai cooking uses this method a great deal. This way, you get the flavor without having the spices in the food. You might not like to bite into a spice while eating. Sometimes bay leaves are added to the garam masala mix to be ground with the other spices. One method which might be tedious but which pays off in the long run is to lightly dry roast all the spices separately and keeps them in air-tight containers. They can then be used in whichever proportion you want to suit the dish you are cooking.
One tried and tested recipe for garam masala is to dry grind the following spices: Black cardamom -8 to 10, Green cardamom -15 to 20, Cinnamon -15 to 20 one inch pieces, Cloves -1 tablespoon, Mace -1 flower, Nutmeg -1, Black peppercorns -1 tablespoon, Cumin seeds -1/2 cup, Coriander seeds -2 tablespoons. Variations to this would be to add dried ginger powder, khus-khus or poppy seeds, dried mango powder, and crushed bay leaves. Remember, garam masala is the heart of Indian cuisine!