I have a new project where I need to set some line 20 snaps and I haven't done snaps before. I watched a few videos showing how to do it and it seems straightforward. I did a couple, rocking the snap setter around in a circle like the videos showed and then straight down, and the posts just get bent over to one side. And the snaps don't snap but I suspect it's because the posts aren't smashed down correctly and straight.

Here's a picture of the result: poor result

Am I rocking the setter around poorly? Not hitting hard enough, or too hard? Do I just need lots of practice to develop the knack or feel for it, like I did with rivets?

One thing the videos didn't show/explain was which size setter to use, and my setters aren't marked so I'm guessing on that.

Help?