Step one, wake your mother at an insanely early hour to ask her if you can use some hangers, rulers, tape, rubber bands and wooden skewers.
Step two, bother her long enough she comes out of bed and helps you. (she also has the day off, but you don't care)
Step three, happily cut up a house supply of hangers, gather enough office supplies to cover all the kitchen counters and stain your hands and clothes with disassembled markers.
And in the end, or after a whole day tinkering, there it is, your crossbow.
Now if you will excuse me I have things to knit, now where is that Ipod so I can count my rows??
New Yarn, new project ooooooohhhhh!!