So you want to roll into a 12 foot vert ramp just like Dave Mirra... Just a suggestion- DON'T START WITH A VERT RAMP!!!! Especially a 12 foot one.
A good height for learning to drop in is on any ramp under 4 feet tall with a platform. Keep in mind that there is 3 typical ways to drop in to a ramp.
Put your bike about one inch from the edge of the ramp- parallel with it. That is your front and rear tire should BOTH be one inch from the edge of the ramp.
Take a big step back with your bike (over a foot) keeping it parallel with the edge of the ramp.
Take the back wheel and pull it another 6 inches away from the edge of the ramp- creating about a ten degree angle towards the ramp.
Put your feet on the pedals and head STRAIGHT forward at a very slow speed. You should pedal about half a crank and then level your pedals out.
About two inches before your front wheel reaches the edge of the ramp turn bars and point the tire almost straight down the ramp and lean a tiny bit forward. DO NOT LEAN AWAY FROM THE RAMP OR BACKWARDS.
You pretty much won't have time to react once your front wheel hits the ramp- The back wheel follows the front wheel and the whole bike will follow where your body leans. So if you are steering down the face of the ramp and leaning forward over your bars then you should have no problems.
Double Pegging In
Put both pegs (on the same side of the bike) onto the edge of the ramp (the coping).
Put your foot on the pedal that is on the inside of the ramp. For example- if the left pegs are on the coping then put your left foot on the left pedal. Make sure that you don't lean INTO the ramp yet.
Take your opposite foot and rest it VERY lightly on the other pedal. Do NOT put any weight on this foot as it may cause premature e-drop-ulation.
When you are comfortable, lean your body forward and into the ramp.
IMPORTANT- Your head, your body, your butt all must lean into the ramp. DO NOT TWIST AND LEAVE YOUR BUTT BEHIND... OR LEAN YOUR BUTT OUT AND LEAVE YOUR CHEST OVER THE BARS!!! Leave the bike behind- it will follow automatically.
As soon as the bike starts to follow- pull a little bit with the bars and point the front wheel straight down the ramp.
Because you are leading with your body everything will follow and you will ride away fine.
When you get more and more comfortable with rolling in you can begin lifting the front wheel over the coping and doing a small bunnyhop over the edge of the coping. This will help in the future when rolling in on vert ramps.
Put your pegs on the coping as with the double peg drop in.
Hold the handlebars normally and make sure the outside pedal is down.
Instead of stepping onto the inside pedal put your inside foot right next to the center of the crank arm right next to the coping.
While leaning away from the ramp put your outside foot over the bike and onto the outside pedal. DO NOT PUT ANY WEIGHT ON IT!
Lift the bike up off the coping and stand on your inside foot with all of your weight holding the bike up. NOTE: Practice standing like this on the GROUND first!!!
Begin leaning into the ramp.
As soon as your are headed into the ramp push off slightly with the foot you are standing on. Do not push AWAY from the top of the ramp but push in a twisting motion so that you help point the bike straight down the ramp.
As your wheels hit the ramp surface make sure the front wheel is pointed straight down the ramp and that your body is forward over the bars slightly.... Lead with your body.
It is very important to realize that dropping in on a mini (a non-vert ramp) has a much different feel then dropping in on vert. A vert ramp is not to different to drop in on- it just FEELS different. The taller the ramp, and the more vert on the ramp, the longer the bike will be in the air before it comes in contact with the surface of the ramp. That is why it is so important to build up to bigger ramps.
In simple terms- there is absolutely nothing different about dropping in on vert- except it feels COMPLETELY DIFFERENT. Start small.