The most important part of a dub 7 is your hips as this is how you initiate the trick. When you come off the jump when you pop you set the trick with your hips as this makes the dub 7 an actual dub and not just a cork 3 to upright 3. Watch a dub 7 that is done correctly and you will see their hips go up (go back so their body is facing up) as they start to spin; you need to drive with your hips off the takeoff. Carving will help though immediately off the takeoff throw your hips up, so at 90 your body is sideways meaning your feet are a bit higher than your head. At 180 your body should be facing the ground as then when you get to 3 your body should be facing the sky. This will keep you corked going into the 2nd three. On your first three you turn your head down as this helps the cork. When you start the 2nd three you want to turn your head to the side but not down This is so at 450 you start uncorking the rotation so your feet are almost in line with your head and it's not as flippy which will help keep it at 7.
Watch this edit and watch the 9 at 0:21, the last 540 of the 900 isn't fully corked and is half upright so it's kind of a dub 9 but not super dipped. Watch his hips off the takeoff and you see they go to the side and not up as much which makes the 2nd portion of the rotation less flippy. Now skip to 3:18 and watch the dub 7. You see his hips go up a bit more and drive, then start dipping to the side which makes the 2nd three corked. Hope this makes more sense when you see an example.