How do I correctly install a dishwasher airgap so it does not overflow all the time?

The dishwasher's drain line usually connects to an air gap mounted at the countertop level on the sink

Dishwasher Repair Guide: How to Clean the Air Gap - ACME HOW TO.com

If I helped you -- please help me -- rate this answer

Down here in Georgia we don't use them. We directly connect the hose from the dishwasher to a trap independently or to the sink waste or disposer before the trap. Loop the hose up and secure it as high as possible under the sink