1.5 litres of water to start with.
Every other ingredient is 'to taste', so add as much or as little as you want.
However, do not leave any ingredients out or substitute them.
Fish sauce (ESSENTIAL, don't attempt this if you don't have it. If you don't have it, acquire some.)
Do not attempt to sniff or research fish sauce unless you are already familiar with it, because you may not want to use it afterwards. ;-)
Kaffir lime leaves
(I rarely have leaves but I keep a stock of limes, so juice gets used more often.)
Galangal (scrape it finely as you would ginger. They are similar.)
Garlic (3-4 cloves. That sounds like a lot, but remember that garlic is godly.)
Coriander leaves (I do not care if you think it tastes like soap, you're using it.)
I also use ground coriander, cos I like it. You don't have to (tho' the seed does taste
different from the leaves).
Lemon grass (if you don't like to eat lemongrass, or it sticks in your teeth, boil the whole stem then remove it before serving.)
Chillis (tiny little bird-eyes are best, but *hot*. I like a mixture of red and green.)
Additional seafood (king prawns/little prawns/squid bits etc.), meat (chicken?), or vegetables etc.
Optional: wedge(s) of tomato, straw mushrooms etc.
1.5 litres is a big saucepan-full. Add all the other ingredients (tho' save some coriander leaves for later).
Reduce the stock until it's to your level of spice, i.e. if it's too weak reduce more, if too hot, dilute.
You can add some rice to make it gumbo-style, and save on having to cook separate rice.
That's all, going to eat some now. :-)