They fence off an area around the hippos, preventing them from getting to any food source. Then they leave food in a big transport cage, wait until the hippos walk in, and close the gate.
Source: http://www.thabamanzi.co.za/Wildlife_Capture.htm (end of page)
@ Fussi: You've said in other answers that you're a *former* zookeeper. Did you come out of retirement or something?