You must first secure the parakeet (holding it tightly in your hands). Next, gently lift one wing and spread it out as much as possible. Then clip the end.
First, get a sharp pair of dog/cat nail clippers. Clip each of the longest wing feathers (primaries) at the base. Leave the secondaries alone, otherwise it can fall over.
You should never really do that yourself. you should let your vet or a vet tech do that job. you could easily clip a blood feather, and have a major emergency on your hands.