Leave slowly.
Very slowly.
I'm talking a year or two.
Start missing a meeting once or twice a month. Same for witnessing, miss one weekend a month.
Keep this up for a few months.
Then miss two meetings, and two witnessing weekends.
Invent an excuse if you need one. Work, family sickness, family sickness, stress/depression, etc. Be vague when questioned.
Keep this up for up to six months more.
Then miss more meetings/witnessing. Become unreliable. Don't turn up for talks. Miss social events if you're invited. Witnessing can be a non-event though you can keep putting in a small report slip if you want to keep the elders off your back. Or stop putting in a report slip at all. Ignore the elder's phone calls at the end of the month.
Keep this up until they're used to not seeing you. Eventually they'll give up on you and ignore you outside the KH. Which is what you want so you can do the complete fade.
Eventually, don't go to any meetings. You won't be invited to social events because the brothers will have forgotten about you. They may avoid you if they see you in the street but that's ok. You are not df'd nor da'd so you can still talk to your family and any close friends who want to stay in touch.
It takes time but you have to give it time to work. And it does work. I've done it.