Honestly, If you are ready for 7s you already know how do to them, but here are a few tips/reminders. Most of all, you have to remember all the stuff it took to land the 540 and do it perfect even if you can do lazy 540s now. My 7s start getting sloppy/inconsistant when I forget the fundamentals.
1.)Use a progressive edge and keep your upper body loose on the approach. If I'm tense, my body position goes to crap in the air.
2.)Milk the pop as much as you can before you spin. If you feel like you need to spin faster--Don't! You don't need to pull any harder than a 540. Work on the pop off the wake if you are running out of time in the air. Don't rush the spinning. Make it very deliberate.
3.)Don't do a 540 to blind. I did them like this for years and could ride away, but you will never get consistent until you give up on that "am I high enough for the last 180," peak at the water.
You'll land this, but don't get discouraged if it is super tough to get consistent. I've been doing them a long time and still have sets where I goof every attempt. No one trick should set the tone of a set so don't get hung up on it (I struggle with this). To quote a pro from years ago: 720s are Fing hard.
Oh, get some video for the Challenge thread and good luck!