After you screw on the two-part lid, your child lifts the cup to his lips and simply sips. There is a slight amount of suction to work with, so the first time they try might frustrate them until they get their first drop of liquid. You might need to encourage them to both sip and suck initially, so that their muscles figure out the exact ratio of each. After that, you’re golden.
And when your kid stops drinking, the cup automatically seals, so it WILL NOT SPILL (despite said kid’s best efforts).