Pour room temperature milk into a big oven-safe dish. It's best to keep the milk at shallow levels, to dry out quickly. I tend to keep it at a maximum of an inch.
Depending on the size of your oven and/or dish, you can pour the milk into several dishes. *
Next, place the dishes into your oven at its lowest temperature. If it can go as low as 120-140ºF/50-60ºC, then that's ideal. Otherwise, select the lowest temperature that you can, using the fan-assisted option.
If you don't have a fan option then leave the oven cracked open slightly, to let the evaporating liquid out so the milk dries quicker.
As the water evaporates, the milk will become pastier and pastier until finally, it becomes a thick paste. Make sure the milk paste doesn't turn brown while drying - that's a sign that the temperature is too high.
Stir from time to time - this allows the water to evaporate evenly, especially when the milk starts turning to a paste. **
Once you get the milk paste, spread it out on a tray or over parchment paper and return to the oven or dehydrator to finish drying the milk.
Once the paste is completely dry, you will be able to easily remove it from the tray/baking sheet. At this point, it will be easily snappable, with no bend.
Grind the flakes into powder in a spice grinder or blender then pass the dehydrated milk powder through a fine-mesh sieve. Then grind the bigger pieces again- that way you'll achieve a very fine powder.
Store the powdered milk in an airtight container. Make sure there is no moisture in it at all and keep stored in a cool, dark place. Milk powder can be kept for months, if not years this way.
* If you want to use a dehydrator then you follow a similar method, pouring around 1 cup of milk per dehydrator tray and dehydrate at very low temperature over a long period of time. I found good results at 57ºC for around 8 hours, stirring occasionally. However, it can take up to 12 hours depending on how much milk you pour on each tray. ** If you stop in the middle of the process, you get condensed milk.There is a third method that could work for those without a dehydrator or a low-temperature oven. Pour the milk into a bowl, using a double boiler method and heat over medium-high heat, stirring occasionally. The water will evaporate into a thick paste. You can spread this out onto a tray/parchment paper to dry out in a well-ventilated area or in the oven at around 65-70ºC, with the door left slightly open, to allow any steam to release.