I have yet to screw up a clone. Here's how I do it.
I count three joints (Y's) down from the top of the plant, then go down to the fourth joint (Y) and snip. Then I cut off the large branch-like leaves. I angle the scissors and cut the bottom of the stem at a hard angle, and I 'score' the stem an inch or so up. By 'score' I mean to very lightly cut, or scrape. You don't want to slice hard. If you do, just cut at an angle and try again, making sure that there's at least three joints (Y's).
Then I dip the prepared stem in water, and then in roottone (can get it at your local greenhouse/garden supply place). I use the cardboard-type eggcartons, which I cut in half. I put the eggtray in the top, and then water the tray a bit. Then I put dirt in the tray, and water again. The water is actually a mixture of BLOOM/thrive/GoGnats/Sweet. This is like mother's first milk is to an infant.......lots of mom's hormones to help things 'root' properly (whether it be brainstem or root systems). I plant the prepared clone in this dirt (just pro-mix) and put it into a recycled salad container (just cover it with plastic, or use one of those plastic oven bags). I make sure it stays moist and I spray it with plain water every other day to make sure that there's no mold/mildew growth. You don't want that.
Making sure it stays moist is essential, but also make sure that it gets air rotation and fresh water to help facilitate growth. I take the cover off of mine daily and spray it with plain water, and then put the cover back on after an hour or so. It took mine two days to start to show roots (gently pull back a little bit of the dirt, but I wouldn't advise moving it very much) and in a week they're rooted pretty well.
PS.......you're not going to kill your plant unless you just start randomly chopping all of the leaves off. Even if you screwed up a few clones, you're not going to kill your plant.