It might not surprise you to learn that supersition ties into your culture as well. Your culture, after all, is built on your terrain and the available resources. Superstitions are much the same way. In order to create a good superstition however, it helps to understand what they are.
By definition, superstitions are beliefs or practices based on beliefs in the supernatural. Often these aren’t based in fact, but they may be based on a false idea of causaution. For example: having a lucky item. If you happen to have that particular item on you when you’re having a good day, you might think of it as ‘lucky.’ Your brain then subconsciously looks for more evidence to back up that idea while disregarding anything that refutes it. In other words: If you think an item is lucky, it will be. The same thing happens in reverse.
Because of that however, things like old wives’ tales tend to persist because lots of people have heard them, and because our brains are looking for reasons to believe them (or not, dependent on your view). Stepping on a crack in the sidewalk won’t really break yours or anyone else’s back, but a common schoolyard rhyme warns against doing just that: Step on a crack, break your mother’s back.
Superstitions also include practices. Things like throwing salt over your shoulder, or knocking on wood are done to ward off bad luck. Picking a penny up face-up off the street invites good luck (some claim this is especially true in regards to finances). Other practices making a wish on a star, or kissing a necklace clasp before putting it behind you. These little rituals are things that you or someone you know might do without putting much thought behind it, a sort of ongoing habit that you almost know doesn’t mean much, but you still do. Just in case.
As I mentioned at the top however, superstitions tie into your culture. In some places, certain colors are considered lucky. Red wedding dresses are signs of good luck in China and India, but a daring and even deadly choice in western cultures. Finding a place to start building your supersition is as easy as lookign at some of your ceremonies. What colors are associated with those ceremonies?
Another place to look at is the animals your created people would be exposed to. Cats are one such example. The Japanese maneki neko is a cat believed to bring good luck to its owners. On the other hand, black cats have picked up an unfortunate and undeserved label of bad luck due to old fears of witchcraft and evil. Similarly, snakes are considered bad, and some practices include nailing a dead snake over the door to prevent illness. Examine which animals your people would deal with, and some of the trouble (or lack of trouble) they might cause. Keep in mind that other superstitions can affect how an animal is perceived: cats in general after all, are supposedly lucky, it’s only black cats that are supposed to be unlucky.
One final place is also in your plants. Knocking on wood is one common superstion, with little known about it’s actual origins. Making wishes on dandelions is another. Plants have a number of uses, from food to medicine, which makes them a prime place for superstitions. Plants that are difficult to grow in a gardens and herb beds might come across as ‘lucky’ plants for those that can get them to seed and sprout. Interestingly this can lead to some curious beliefs as certain plants should never be given away: instead have a friend ‘steal’ them from your garden.