I will admit that I have never been to Romania and will base my answer on the guess that "smântână" is a loanword from the Slavic "smetana" - which is highly probable given my knowledge of the mixing of language and culture on the Balkans, and your description of the final product.
The English word for it is "cream". Typically, cream is in the range between 30% and 50% fat, although the transition to milk on the lower side and butter on the high side is fluid. Outside of pastry chefs, most people nowadays are familiar with the 30% to 33% "whipping cream" sold in supermarkets, and may be surprised to encounter products with much higher fat and hear that they are still considered "cream". What you want is probably 45% and above, making it "double cream".
The way of obtaining a thicker cream is to simply increase its fat content. The fast way is by a separator, the slow way is by letting it sit. The starting product can be milk, raw or not, or already-made cream (by separator or by sitting), as long as it has no stabilizers (look on the label for the words "stabilizer" or "carrageenan"). A separator is perfectly suited to making thicker (higher fat) cream, you just have to let it run longer than for the whipping cream sold in supermarkets.
So let's assume you wish to start from raw milk. What you should do is:
- put it in a vessel of your choice. Preferably one with a wide opening (like a pot) rather than one with a narrow opening (like a bottle). Make sure you can close the vessel reasonably well (a normally fitting lid on a cooking pot will do).
- put the vessel in the fridge and let it sit undisturbed.
- when the smântână has built up, gather it.
To go to your points one by one:
- the temperature: Do it in the fridge. The traditional way of doing it outside is unsafe.
- the time to sit: Several days. It's impossible to give you an exact number, you have to gather it when you consider it good enough
- the way it is traditionally separated: By any way you can accomplish the task. You can gather it with a spoon, or with a slotted spoon, or you can attempt to pour out the milk only without disturbing the cream.
- "Anything else I should know to increase the chance of obtaining a thick, highly viscous cream which doesn't taste sour"
- if the stuff you get feels too liquid to you, you might want to switch to a multi-step process where you gather the top layer roughly, with a bit of milk still in, transfer it to another vessel, and wait for this slightly-concentrated cream to cream up even better.
- instead of starting with raw milk, start with whipping cream from the store and leave it to sit in its original sealed container until shortly before its best-by date.
Note that there isn't that much fat in milk, and the traditional method is also rather inefficient, so about 50 g of cream per liter of milk would be a great result. Don't wait too long in the hope to see more cream building, the time you can wait is bound by the milk spoiling.
You also asked if it's not cultured. Cultured smetana is a different product (I suppose it exists in Romania too, but don't know the term for it). People talking and writing about the two don't always bother to mention the qualifier "cultured". I don't know if you may actually want the cultured product, but to have happened upon descriptions of the non-cultured one and to have mixed them up.
If you want cultured smetana as widespread on the Balkans (it has a different texture and is more sour than the noncultured one described above, but without being spoiled), you have to obtain a L. bulgaricus based yogurt culture and use it to culture whipping cream (not milk), using the exact process prescribed for making yogurt with it. You can obtain the pure culture - Laboratory Genesis ships throughout Europe, I believe - and it will have instructions. Or you can buy yogurt labelled "Bulgarian yogurt" in the supermarket and use it as the starter - here, you can follow a standard yogurt recipe, but remember that L. Bulgaricus is thermophilic, so you should incubate at 43-46 C for best results.
And now for the difference to the creme fraiche. The most traditional smântână (the "sweet" one made by sitting) does indeed have some bacterial activity in it, it is made by leaving the milk at room temperature. But you have no way to control the proportion of lactobacillic bacteria to pathogens multiplying by it. The texture and smell (being spoiled) is a very rough way of recognizing the worst batches, but even in the properly prepared ones, you will be consuming significant amounts of common pathogenic bacteria, and with some of them, you will also be consuming less common, but more dangerous ones like listeria. So, if you choose to do it that way, be aware that 1) you have a rather short window of consuming it between the cream forming and spoiling, and 2) when you consume it (before it has visibly spoiled) you are already deep in what is considered unsafe by modern standards.
The difference between modern cultured smântână and creme fraiche is not the use of a starter, it is the culture used in the starter. Creme fraiche is done with a different species of bacteria in the starter.
