You can find an example setup for 2 drop water collision below. In this setup, you should put your camera to BULB mode and trigger your flash to eliminate the shutter lag of your camera. If you don't have a camera connection cable, you can start the long exposure manually before each shot.
1st Drop: Duration: 50ms
2nd Drop: Duration:50ms - Delay:100ms
3rd Drop: Duration:0 - Delay:0
4th Drop: Duration:0 - Delay:0
Trigger: Mode: Flash Delay:200ms
Height: 25 cm (from the water surface to the dripper)