^ Ground level ^ 1 level down
In the top-left of the images is a murky pool, in the bottom-right is the reservoir (ignore the little 1x1 murky pool at the top). Make the reservoir by channeling on the ground level. Mine is 7x7 (with the top left square cut out, so there's a 1x2 gap between the pools to fit my screw pump) for lots of water storage, but you only need 3x3 or so really. The walls are there to stop water overflowing when the pump is used, because if the water overflows and touches the murky pool, the reservoir becomes stagnant also.
1. chop trees
2. wood workshop
3. 1 x wooden giant corkscrew
4. 1 x wooden pipe
5. masons workshop
6. 1 x rock block (i assume you have rocks)
7. press 'b' then 'M' then enter to build the screw pump. The
dark green tile is placed directly adjacent to the
reservoir (inbetween the walls in my case) and the
light green tile is placed directly adjacent to the
murky pool. I think it's 'u' 'm' 'k' 'h' to change the direction of the screw pump, interface should tell you anyway. Press enter when it's in the right position.
The pump draws water from the tile below the tile next to the
light green tile, and deposits the water in the tile next to the
dark green tile, which then falls into your reservoir.
8. Press 'q' and hover over the pump, then press enter to start it up. Repeat to turn it off. Make sure you have at least 1 dwarf with pump operating enabled. Just don't forget to turn it off when your reservoir is starting to get full!
Edit: Bear in mind that dwarves prefer to drink alcohol than water, of course