As mentioned, it just takes a lot of practice and the key is to use the correct length of rod so that your peens look the same on both sides. When I started I quickly learned to not leave too much material hanging over or your peens are going to look bad. I usually cut my rod a little longer than need with flush cutters. I use a file to sand the end smooth, insert through the washers and scales and gently tap that end to get a peen started. I then flip the razor over to the other side and cut the rod to close to the amount that I want hanging out to get nice sized, but not too big of peens. Use a hand file to sand that side smooth as well and begin peening, back and forth with light taps until it is snug.

I also polish after with a quick trip to white rogue and then some mothers aluminum polish. After you've done a lot of these it becomes second nature.