Hey thanks for these questions! It can be quite difficult, since the game has many variants:
Have a good cook, which IIRC uses the house keeping skill. This will affect food quality, that has an impact on happiness. Cleaniless also affects this.
The happiness of your servants is quite a complex thing tho, so I will try to adress some basics:
It may be affected by their upbring: if they have a rich, noble, etc. upring, they will hate to do anything beneath them, which is almost everything except few tasks, like courtier or counselor, which are forbidden for slaves anyway.
This also affects a lot their lifestyle: If they have a poor upbring, they may be used to cheap food, sleeping in barracks, etc. but if not, then they will definitely complain and hate it.
Also, they may refuse to do their tasks because they have low obedience, or fail them because they dont have enough skills (which IIRC they grow passively over time), this means they are likely to be punished by your slave master, which in turns will make them unhappy. Also, how unhappy etc. will depend on your slave master skills, altho i dont recall if he used security, persuasion or what.
Also policies matter: forcing them to be silent, or deny them stuff will make them unhappy, but giving them free time, private baths, etc. will enhance it, altho their maintenance will be higher
Other stats like their obedience and will power may affect this too.
Overall, unless you actually treat them well/accordingly to their former status, they will either be hard to manage, so is not about just having 5 slaves or workers, but to also be more selective.
As a last note I worked in this feature a long time ago and havent had time to go more in detail about the feedback you receive (aka the reports, which change daily, but dont have too man variations), and is complex to the point i dont remember everything unless i go back to the code, but I will work on it again soon and keep polishing it