Have fun with these. Your imagination can go bonkers.
First you need to get line drawings of butterfly wings. You really only need the outline so cookie cutters would work. Some of the nature butterfly books have great drawings. Then there's always the internet.
When you have your outlines tape them down securely and tape waxed paper or parchment on top.
Pipe the outline of the wings. Don't worry too much about inner lines -- maybe a line across dividing the wing. I have found that having more room for the thinned icing makes it easier to decorate.
When the outlines have dried a little, thin your royal icing to flow consistency. Don't make it too thin but you want it to flow.
Pipe the thinned icing in stripes or concentric circles and use a tooth pick or pointed bamboo skewer and draw through the lines on the icing wiping the stick between pulls. Relax and have fun and let your imagination run wild. They aren't supposed to look like real butterflies. Let the wings dry flat at least 12 hours or longer if you can.
Use regular strength RI for the bodies. I used a strip of parchment about a half inch wide in my flower former, piped the body on that and pushed the wings in letting them rest on the sides. You can also prop them up with cotton balls. Stick flower stamens in for antenna and let them dry until they are completely dry. I got in a hurry and pulled a wing off. Big bummer!
These guys are very light and fragile. I handle them by the antenna whenever I can because they break easily. But they are tasty!
Time for me to get myself that tip 
Wilton carries them as do many other suppliers. You can get them in black but I couldn't find them so I painted them with food coloring.
Thanks to everyone for their comments. I had bought the outlines for RI butterflies with all the inner lines like you see done in all white on cakes, thinking I would do them and send them to my DIL for my granddaughter for her birthday in June and decided to play around with just using the outlines and flooding the wing. I'm glad you like them.
Julia1963, I am not a professional either. Just a 70-year old great-grandma who really loves doing this stuff.
Thanks for the tips. My DD just saw them and wants them on her birthday cake this year. It will also give me practice for my first wedding cake which I have booked for Aug 09, theme being hearts and butterflies.
thanks again.
Nikki
Those are gorgeous butterflies!! I've seen them done in more whimsical pastel colors but those are outstanding!! Very nice work! 
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