I think the easiest thing would be to set up simplistic aqueducts and flowing latrines.
We know water wheels were commonly a thing, so we could start normalising the construction of water wheels to raise fresh water to a simple wood/earth raised waterway, ideally covered, but eh - that travels into the heart of the village, and then back out again. Inwards, you've got fresh, relatively clean, water - and on the way out you have a self-emptying latrine.
Don't get me wrong, it seems like something better done in partnership with a mill, but if you're close enough to a river, it makes sense to do. The big problem is that it relies on much more construction and earthworks than I expect would be used otherwise.
The idea could just go to full on aqueducts like they'd have seen historically, which could see multiple villages fed and flushing on a single system. Heck, considering it is fundamentally a core piece of infrastructure, that'd need paths to allow maintenance, and it would connect to every village - you could well see it be tied to the castle of a local lord if that approach was to be taken.
The smaller systems are a bit of work, anything bigger would be MUCH larger. So I think you'd also have to figure out a way to have the whole program make money, which to me screams "irrigation".
But yeah, TL;DR - develop a combination of irrigation, freshwater, drainage and sewerage system using basic water wheels and controlled water flow.