Cisterns are pretty easy.
1) Dig a down-stairs near a murky pool. Leave 2 tiles between the pool's edge and the stairs.
2) Go down 2-3 Z levels, then dig a sideways corridor about 3-5 tiles long. Then a room that is 3x3 or 5x5.
3) Channel out the floor of the 3x3 or 5x5 room. The size of the room in total squares should be equal to about 3/4 the number of tiles in the Murky Pool.
At this point you have a choice. If time is of the essence, just channel out the 2 tiles between the pool's edge and the staircase and flood your cistern. It should fill up the lower level with 7/7 water and the upper level with 2/7 to 4/7 water. Once the pool is drained, put a constructed wall in place of the murky pool's edge, then build constructed floors over any other openings (such as the stairs and the 2nd tile that you channeled out). Or you could hide the stairs under a constructed wall.
If you have more time/labor, then you can use a screw pump to move the water from the Murky Pool into your stairs. You'll need to direct the output of the pump into a U-shaped piece of constructed walls (5 pieces of stone block for the walls, plus a wooden corkscrew and wooden pipe section built at the carpenter's workshop). And a dwarf with Pump Operator labor. Moving the water in this way removes the "stagnant" tag and gives you a clean water source.
If you have a *lot* of time/labor then you can make the cistern 3-4Z deep, narrowing as it gets deeper and smooth each level before filling it. But unless you have 3-4 engravers, you probably can't get this done before the first winter.