There's no real consensus on what to call the falls; some call them the Cat Cat Falls (Cat Cat is just the Vietnamese transliteration of the word cascade) while others call them the Sapa Falls.
To get here, just head down Fansipan Street and keep going. Shortly after Sapa Eden Hotel you'll reach the ticket booth where you'll be asked for 40,000 VND. The road creeps along the edge of the mountain with some beautiful views.
After a few downhill kilometres, you'll find the entrance to Cat Cat village marked by a gate, easily spotted in a cluster of cafes and restaurants. Proceed down the steps, through the rice fields, to the village and falls. There are no shortage of cafes, stalls and playing children along the way.
You can take a dip by crossing the bridge near the falls and sliding down the embankment on the other side. These aren't the most spectacular falls in the world, but seeing them is a very pleasant way to spend a few hours.
On the other side of the bridge is a Fairy Stream which can be visited if you've got the legs to add it in, and it's possible to extend your walk further with a trip to Xin Chai village. Some agencies charge for this trip, but that is sort of unnecessary as it's just about impossible to get lost doing it on your own.
To finish up, continue past the falls to along the path to the entrance gate, about a kilometre further on. From there, you can walk back, or take up one of the motorcycle taxi drivers on their offer of a ride — usually about 50,000 VND per person. If you're on a motorbike and not keen on walking, blow past the Cat Cat gate and continue down to the entrance gate where you can park in one of the many cafes offering parking for a fee and walk to the falls, a kilometre each way.