I remember when I was first figuring out how to into wells.
I first decided to make a well when I realized that injured dwarves won't drink booze. One of them died, and a lot of dwarves liked him. His death ended up ruining everything via tantrum spiral.
So the next fort, I decided to make one, right? But I had no idea how wells work. There was no river or brook in the embark area, only lots of underground murky pools. I didn't think of going down the cavern layers yet, mainly because I didn't know they existed. I made an aboveground well and I connected all the pools together via channels and pumps. It was shitty and everything was stagnant. Eventually a loyalty cascade ended that fort.
My next fort didn't fare better. I decided to embark near a brook. I channeled into the brook one winter and I made my well, but I screwed up and I didn't close the breach in time for spring. The entire fort was flooded. On the upside, I trapped some elves and they went insane. It wasn't so bad.
Come the next one, I thought I'd be able to do it. Again, I embarked near a brook. Again, I channeled on the brook, but this time I first diverted the water into a reservoir, taking care to make it three z-levels deep to avoid muddy water. I then made the well, and everything was good. At first. Remember that the reservoir was connected to the brook directly. Eventually the well overflowed and the entire level was flooded. Thankfully, I managed to block it before everybody drowned. The fort ended when clowns came in.
This happened for a few more forts: In the next one, I forgot to install a floodgate. Then somebody tampered with the lever. Then I accidentally everything when six dwarves fell into the reservoir and I was trying to fish them out.
It took me, what, ten forts? But now I can say that I can do wells. Yay.