Determine the area where you want to place the towel holder.
Mark the spot on the wall where you will insert the butterfly bolt, using a pencil.
Measure the end of the butterfly bolt to ascertain what size drill bit you'll need. Choose a bit that is slightly larger than the diameter of the bolt.
Place a small piece of masking tape on the wall, if it's plaster. This protects the edges of the hole as you drill, and prevents them from cracking or crumbling.
Drill a hole at the location you've marked for the toggle bolt. Remove the tape from the wall.
Insert the screw through the hole in the towel holder and then into the small hole in the center of the toggle bolt. Fold the bolt together. Place the screw in the hole in the folded end of the bolt, from the inside of the fold. Make sure the screw is screwed into the bolt securely.
Push the folded bolt through the hole in the wall slowly, until you hear or feel it spring open on the other side of the wall.
Pull on the bolt with one hand to create tension against the inside of the wall, and use your drill to screw the length of the bolt into the wall. Hold the end of the towel holder in place against the wall as you're tightening the screw. Eventually, you'll feel the toggle bolt pull firmly against the inside of the wall, holding the towel holder in place.
Tighten the screw until you can't turn it any more.