-Start! Order your 6 drones to mine from the nearest mineral fields to your hatchery.
-Build one drone and rally it to your opponents ramp.
-Build another drone once your first drone is about 1/2 of the way to your opponents base.
-Use one of your original 6 drones (the one mining furthest away) to create a spawning pool as soon as you hit 200 minerals.
-Build another drone and rally it to your opponents ramp as soon as you get another 50 minerals.
-Wait until the spawning pool finishes, then build zerglings and overlords as needed. Make sure to rally them to where the action is!
Your goal is to delay their barracks until your zerglings have arrived and prevent the terran player from walling off at his ramp. Your drones that you rallied at the beginning will need to damage the SCVs while they're building, avoid direct combat, and stand in the way of where your opponent wants to build. Once your first wave of zerglings (4) have arrived, your object is control. Your opponent is going to have a lot of unspent minerals, so don't just charge straight into their mineral line. Pick off SCVs as you are able, protect that ramp and don't let him construct any more barracks. Wait until you have enough reinforcements and then thrash his SCVs and whatever uncompleted buildings he has. At this point, your opponent will either surrender or lift off. If he does lift off, just follow his buildings around while you tech up to mutalisks and watch the expansion areas carefully.