(b) (l), needs a block, bucket, chain or rope and a mechanism (last one not sure)
find a water source. It works with water in caverns, but this is not always deep enough to prevent the water being a bit muddy
by careful digging and channeling you can link up many murky pools to collect rain water - this water is stagnant and not really drinkable, so let it flow to a collecting place and then pump it in the cistern for the well with a screw pump. Streams and brooks can be breached from the side or below to create a continuous flow. The water is usually drinkable, unless you linked a stagnant pool to the river or some other reason. rivers and bigger can be treated the same way, but lower, if you prefer.
the river has infinite water. This means that it will keep flowing untill it has filled everything up to the level of the surface of the river. Make sure you have a way to shut the flow off, or you'll flood your fort. If the river is several z-levels deep and you breaching near the bottom, the water will be under preasure and flow much faster
Remember that a well over caverns creates a potential access to your fort, just as the opening to a river does. I'm not sure if a well can be destroyed from below, but a building destroyer coming from the river could be a problem if you only have a floodgate.After destroying the floodgate it will have a free run into your fort, and now there is nothing to hold back the water, and nothing will be done about it because, surprise, there's a monster scaring everyone away