A request has been received for this. I wish I could give measurements, but keeping with old world traditions, I measure with my eye. But here's how I make malteds.
I start with chocolate ice cream. I can't emphasize this strongly enough. If you're making a chocolate malted, this is a requirement. My prefered chocolate ice cream is Haagen Dazs. I once couldn't find any, so tried chocolate chocolate chip. This does not make the best malted, but the mound of ground up chips at the bottom of the mixing container was quite tasty. But I wouldn't try to get fancy with chocolate fudge ice cream or some such. The flavor wouldn't be right.
I soften the ice cream a bit in the microwave, just to make it scoopable. Too soft and it won't mix well, too hard and it's impossible to scoop out. I use enough spoonfuls to fill the metal container somewhere near the indented line inside. Please note that I use a Hamilton Beach DrinkMaster, a niftly little appliance well worth the price. My malteds improved greatly when I stopped using blenders. The glass canisters that blenders use as well as their mixing action differ sufficiently from the metal container and mixing action of the DrinkMaster to affect the flavor of the malted. That might sound nuts, but having grown up with malted made in DrinkMaster-type appliances in old-fashioned coffeeshops, I can taste the difference.
I pour in milk. Not a lot. Usually enough to be visible above or in between the mounds of ice cream. I can add in more as the concoction is mixing if the consistency seems too thick. But I don't like it with too much milk because that can make it too thin and also dilute the chocolatey yumminess. But you don't want it too thick, because that can leave you with a chunky malted and that won't do at all.
I add 4 heaping scoops of Carnation Chocolate Malt. Carnation makes regular malt, but I prefer the chocolate malt which is usually more difficult to find. We stock up when we see it in the supermarket. It's usually found with either the powdered milks or in the freezer section with the ice creams, on a shelf with hot chocolate and such.
Try to not fill the container too high. This is an open top appliance and you don't want the liquid concoction to spew out all over the kitchen. This is the VOE (voice of experience talking).
I do not, as many people do, add ice to the mixture to keep things cold. The ice cream and milk should be sufficiently cold on their own. Ice just dilutes the liquid too much for my taste.
You should be able to squeeze 2 full glasses from this mess. The glasses should have a nice, thick bubbly head. Drink through a straw. It'll taste better.
The key here is to experiment a bit. Don't expect to get the proportions right at first try. After some practice though, you should be able to develop the ability to eyeball the amounts and know before mixing that they're going to come out "just right." So have fun playing and remember that it's a personal taste thing. You might want it a bit less chocolatey than I do, or you might prefer regular malt to chocolate malt. I might think you're nuts if you do, but hey, that's my problem. :)