This is your aerial prep for a left aerial. Hop on your weaker leg, and swing your arms up above your head.
The next step is a transition step, and you do not hold it. The photo looks funny, but in real time, it takes about half a second to do :p
From your prep, step onto your other leg. You want your shoulders up, your bottom leg bent. Fling your free leg over your head as fast as possible while jumping off of your bent leg.
In an aerial, you're actually pivoting around your shoulders, not your hips. Because I kept my shoulders up, my head is about 2-3 feet above the ground. Continuing on next page...
From here, bring your first leg down quickly. Personally, I think about bringing my hip and making it meet my ribs. Land on a bent leg to absorb shock and save your knees for the rest of your life :p
As with your takeoff, your shoulders will swivel in your landing. Land on a bent leg, and let your second leg pull your torso to a standing position.
The next few videos are just a few different ways on working up to an aerial. In this one, you put your hands down and halfway through take them off. This works on your landing.
This way is called a dive cartwheel, and you are trying to keep your hands off the ground for as long as possible.
After you've gotten some practice, work on keeping your hands off during the whole aerial! All it takes is height in your aerial and self discipline.
Once you have mastered your aerial from a run, try going from a standstill :) good luck guys! Be careful!