You need to cut off the top and the bottom of the bottle. I use a knife to "stab" a hole in the bottle in the area I am going to be cutting, then I used kitchen shears to cut the plastic.
After you have the top and the bottom cut off, you need to cut a line down the center of the side opposite the handle. (Most bottles have a seam here anyway, so it is easy to follow the line to cut). Then you can push the sides of the bottle open like this:
The handle becomes the body of the butterfly, Use your imagination and cut the wings in whatever shape you would like. You can save the plastic you cut off to make shapes to glue on later.
Some detergent bottles have easily removable labels. Some do not. For the ones that do not, you can leave the labels and cover them as part of your design. You can paint them with spray paint made for plastic, or you can cut another large piece of plastic and glue it over the label.
When you cut the bottle you can also shape it near the body and "head" Some of our bottle butterflies have antennae, some do not.
The decorating possibilities are numerous. Our crew opted to use colored foam stickers, foam sheets (they could cut shapes themselves), sequins, glitter, and permanent markers. We used a hot glue gun to attach everything but the stickers. (You can buy glitter glue sticks for the hot glue gun that work great for decorating!!!)
You can easily hang the finished products by punching a couple of small holes in the plastic. We often punch a small hole in the handle and place it on a stick to use as lawn ornaments outside. (These last two photos are of a couple of butterflies that have "survived" since we made them last year, the weather has faded them a little and some of the decorations have disappeared, but they are still part of our rock garden)