I have a kegorator and I usually make 5 gallons at a time.

Clean/sanitize a keg and fill it with water and add a crushed up campen tablet (potassium metabisulfite) to sanitize and remove chlorine from the water.

I use about 3.5# of corse ground of whatever sounds good at my local coffee shop.

Clean/sanitize the keg, purge the keg with nitrogen.

Add the coffee to a muslin bag and put it in the purged keg and do another purge. Then transfer the water from the first ket onto the coffee.

Let sit for three days, I've found this is the sweet spot for me personally.

Then I transfer it back to the first water keg that had been sanitized and left under pressure with nitrogen through a 10 micron filter to remove any residual coffee grounds.

I have really soft water where I live so I don't mind using city water but I'd love to play around with the water chemistry to really refine it. I haven't looked into that at all specifically for coffee though.