How to build a smelter

Happiness goes down by 2% for every kitten you have after the fifth kitten (so the sixth kitten will drop it by 2% from 100% to 98%)

Happiness is the efficiency of your kittens. For example, a farmer (without any upgrades) normally will yield exactly 1 catnip per tick, but at 98% happiness they'll only yield 98% of that (0.98 catnip per tick)

Hunters can bring in items from the forest (I've only seen fur and ivory), each will increase the happiness by exactly 10%, cumulative (getting both increases by 20%). The amount of each of them only affects how long the bonus lasts, not how many the happiness bonus is

EDIT: Also, Amphitheaters reduces unhappiness by 4.8% each, which means that it will negate the happiness reduction effect by overpopulation, but can't actually increase happiness (so if you have only ever had 5 kittens, it will not increase the happiness)

EDIT 2: In reality, it looks like that as of v 0.9.6 Amphitheater does not actually reduce unhappiness by 4.8%. I have 18 kittens for a projected happiness level of 74%, unicorn raises it to 84%, yet 7 amphitheaters only brings it to 93%, about 1.8% each. I'll see if I can find more on this

There are three main ways to get coal. The first two you'll get are the Coal Furnace and Deep Mining upgrades at the workshop. These will let you get small amounts of coal for each mine (0.003 per tick, 0.015 per second) and running smelter (base .005 per tick, .025 per second).

But, by far the best source of coal, is unlocking the Geology technology, and assigning Kittens to the Geologist job. Geologists are worth .015 coal per tick , equivalent to three smelters, or five mines. And kittens can improve at their jobs; a Master Geologist is earning about 0.026 coal per tick, which is over 0.1 coal per second each.

There are also quarries, which each give a +35% boost to mineral production, as well as another 0.015 coal per tick. They won't improve on their own the way Kittens improve their skills, but they still help a lot.

There are also several upgrades at the workshop related to coal which can help. Pyrolysis will increase coal gains by 20% from all sources. The Geodesy will increase Geologists' base coal per tick to 0.0225, making even a Dabbling Geologist worth four and a half Deep Mining mines. It also allows Geologists to produce a small amount of gold while they work.