I assume you mean the people you already have living in your settlement and not people you are trying to recruit.

First and most important they need a house allocated to them. The higher quality the house and the more upgraded it it, the greater the boost to their mood.

Next they need access to food and firewood. You can put this in their house storage directly but that gets tedious to micromanage. Best way is to build food storage and resource storage, and stock them with food and firewood respectively. All villagers will automatically take what food/firewood they need from these buildings.

HINT No need to get fancy on the food at the start (mushrooms, roast meat, etc will get you by) but at first opportunity aim for potage as its a good quality food thats easy to make (meat and cabbage). Apparently they will burn sticks/logs/planks if there is no firewood, but since its so easy to make I can't confirm this.

Another boost comes from giving them a job. The higher their skill in a particular task will give them a bigger boost when performing that task. e.g. someone with hunting skill of 3 and crafting of 1 will be much happier working as a hunter than working as a craftsperson.

Another boost will come when you put a male and female together in the same house. After some period of time they will Marry, and then eventually have a child. This boosts both their moods. I can't say exactly how long this takes as there is probably a random element to it, but I have heard that if the age difference it too big they take longer to settle into a relationship.

In your diplomacy skills you can spend points in abilities to improve the overall mood of your villagers. I personally don't bother as I'd rather put those points elsewhere - like higher selling price, lower purchasing price, etc.

Lastly, at each seasons end a random event can take place. Some of them result in a positive or negative effect to the villagers mood, but it only lasts the duration of the following season.

Since its early access more mechanics might be added later, but for now thats all I can think of. Hope that helps :-)

PS I forgot to mention that if the house they live in is damaged and needs repair, then their mood will drop until you repair it.