A few months ago a friend brought some spices back for me from a trip to Indonesia and since I already had them I started making my own masala chai. It's not that hard.
One way to start is to Google something like "how to make masala chai." You would find articles on just that, like this one, or this one (it's paleo!), or this one. Of course chai actually means tea, so normal for them to point out saying "chai tea" is redundant, but you still can if you want.
They all vary from a basic original recipe, with lots of claims that one version is authentic, or easier to make, or that they've been adjusting it for years and prefer it a certain way for the best reasons. What I'll write is only my own take, and I'm not Indian, or a tea expert (really!), so take it for what it's worth.
-nutmeg (optional per some recipes, but I'd add it)
-cardamom (basic to all ingredients, but really could skip it since flavor profile is close to nutmeg)
-ginger (kind of critical)
-black pepper (or not; it's your tea)
-milk and sugar (could be honey or brown sugar)
Other basic: you see clove a lot, and pretty much all recipes will add black pepper, and a lot reference star anise. I tried star anise and could take or leave it since it's a strange flavor; clove would be good but I've not got around to adding it.
Other not so basic: bay leaves or basil show up in some recipes, or anything else. I've tried it with tamarind (prepared for use as a spice) and lemongrass added but those are quite non-standard.
The easiest recipes call for either a five minute boil for the ingredients (or maybe 10) or just for steeping.
Prior to that step if some of the ingredients aren't already ground then preparation would be required: cardamom comes in seed pods, ginger is a root (obviously), cinnamon might be in the stick form yet, etc. Some recipes call for use of a spice holder muslin bag, although straining could cover that, with the worst case being some spice dust in the final tea.
To keep it simple you could just use powder versions of all the ingredients, mix it, and steep. From tea brewing habits I would steep it multiple times for about 3 minutes each but one five minute steep would get you to drinking tea. One might drink it without milk or sugar, if they like, but I can't imagine why.
Amounts: this isn't really a recipe yet, right, no proportions. Those other links all say what they think is the right relative proportion, and yet another says they've nailed the best possible mix, but I tend to just make it one way, then another, and keep shifting it, never really measuring at all. Crazy, right? I can see why a lot of people would want more consistent tea, so go that route if you like, be normal and measure.
don't be afraid!
make your own masala chai!
Tips / tricks:
Loose tea: Seems natural to me that everyone would have lots of loose tea at their house but if not using a standard black tea bag would do. But really, why not pick up some passable loose black tea. The normal standard would be to use a tea from Sri Lanka (Ceylon, the old name for there), and those tend to have a good bit of astringency, but that doesn't matter if milk and sugar are part of the equation; they cut that bite back to nothing.
Blended teas might also combine teas from different places, or there is a lot of tea coming out of Kenya now. Living where I do (Bangkok) I've bought ordinary grade black tea from Thailand, Malaysia, and Cambodia, and tried lots of types of teas from lots of places. There's no reason why a different tea couldn't work, or a mix, and paging through references eventually someone will mention using green instead, but black tea just makes lots of sense.
Ginger seems the next most critical ingredient. It would be possible to use ginger powder or ginger tea bags but fresh ginger would be a nice touch, with a different flavor. It would be easy to grate some right from the root into the mix or I've just cut chips off one instead (we always keep ginger root around; I live in Thailand). It wouldn't hurt to add two different types of ginger if it's handy, both powder and fresh.
Pepper, nutmeg, clove, star anise: like everything depends on preference, right. For me part of the experience is making different types, as one would with cooking. I love using cloves to cook so it's odd I've not been adding them, but I'll get to it. Star anise takes over a bit, even in strong flavored blends, and it has a strange sweetness, so someone might either like or hate it. Pepper might be crucial to some, the key ingredient, and better off left out to others.
Boil versus steep: trial and error might root out a preference but I'll say a little about this. In general no one would ever boil loose tea to prepare it (in general, but people do crazy things), because the essential oils of the tea--where flavors are--would tend to evaporate.
If someone was using fresher spice ingredients in a rougher ground form then a boil might be required to pull out a lot of the flavor, but then I'm talking about making an easy DIY version, not the optimum some Indian cafe is aiming for. So steeping alone really should work.
Freshness: goes without saying but adding any ingredient that has spent a couple years in an opened spice bottle will more or less ruin the whole tea. Black teas are as durable as any kind (except pu'er and some oolongs--special cases) so if it's been hanging around for a couple months it should be ok but the right packaging is still critical.
Ideally all the ingredients should be relatively fresh and converted from the whole spice form just before making the tea, but you work with what you've got. Someone could go the extra mile without buying a spice grinder by experimenting with a food processor or blender (for example), but bottled ground spices would work.