Just know that at the end of the dry season and beginning of the rainy season, the water level is often very low and the waterfall is sometimes dried out. The best months to have a real flow of water are generally from July to November.
You can take one of the red cars (songthaews) to come to Huay Kaew Waterfall, it should cost you 30 bahts per person (more expensive if you privatize the red car). To go back to the city you can join the main road and hail one of the red songthaews descending from Doi Suthep temple but all will not stop especially if you are numerous because they are usually already full. If you are more than 2 people it is probably better to have your own transport or to ask to be dropped at Chiang Mai Zoo and return to the Zoo or University a little lower down on the road to take a red car from there.
There are oftem more people on weekends, locals coming to spend a few hours there with their family and friends.