First, get a Parachute (one can be purchased from Armando’s gun van if you have completed the “High Dive” mission in Yusuf’s mission line) and find a helicopter.
Before entering the helicopter, be sure that your Parachute is equipped (it will be visible on Luis if it is). If it isn’t, use the D-pad to cycle through your inventory until you are at the Parachute and press RT to put it on.
With a Parachute equipped, get in the helicopter and hold RT until you’ve reached maximum altitude. To ensure that you’re as high as you can go, look to the altimeter on the radar. Fly your helicopter over a section of the city that’s relatively clear of buildings. Then, press the Y button to jump out of the helicopter and begin freefalling.
To unlock the Adrenaline Junkie achievement, you must press the A button to activate the Parachute as late as possible in the jump. This means waiting until the altimeter is closing in on bottom of the radar. Note the altimeter in the video when the cord is pulled. This video is from our unofficial game guide. You can view the full guide here