How do I "evaporate" milk?

Very easy. All you want to do is to remove the water through heating. As you are not going to can it, you don't have to sterilize it afterwards.

The only concern when evaporating milk is to not end with a layer of scorched milk solids on the bottom. First, start with homogenized milk (you don't want to risk undissolved fat swimming on top of it). If you want the pure milk flavor, heat it to 70°C and wait until you are left with half the initial volume. This takes several hours, depending on the quantity of the milk. My great-grandma usually did it overnight.

If you don't mind a "scorched" taste, you can do it much quicker. Take a very big pot. It should be at least 6-7 times higher than the milk depth, because the boiling milk will foam up a lot. The width is up to you, but the wider the pot, the quicker it will go. Bring the milk to a vigorous boil. Use a setting which is just hot enough to sustain boiling, too much heat will increase the scorching. Stir all the time. You need maybe 10 minutes of stirring, again depending on quantity of the milk and pot width. You are aiming for the same rate of evaporation - you should be left with half the original volume, or somewhat less.

You don't have to add sugar to evaporated milk, the sweet taste is caused by the heating. Especially the boiling-evaporated milk is quite sweet, because you get some caramelization.

I've heard that some people simply put milk on a large pan or container and leave it there for some hours to naturally evaporate without heating it up or anything. Did anyone here try that before?