How can I make the second wing like the first?

Try to draw a light line with a ruler. Be careful, though, the pencil will only erase what it can. Anyway, sketch a line lightly in the middle of the butterfly. Once you have done that, try your best to just look at the other side, and again, sketch lightly because you are not sure if you can do the other side right in just one try. So when you finish the other side of the butterfly, see if you got it right. If you didn't get it right, erase your errors, and keep sketching until you get the other side right. If you get it right, however, use the pencil or a fine black marker and outline the lines of the butterfly. And in my opinion, I think that strategy is the simplest one. Another way is to use tracing paper or other thin paper, put it over your butterfly and trace the first wing. Cut that out, line it up on the other side, and trace it.