Im positive you know what a tap looks like. Just make the bolt look like that. Cut one groove down the length of the bolt. Make the groove deepest at the start of the threads and make it shallower toward the head of the bolt. Also, the groove should stop about 1/4" from the end of the threads.
http://www.machinemart.co.uk/images/library/product/large/04/040210261.jpg
The only difference on that one is the channle isnt tappered.
http://www.machinemart.co.uk/images/library/product/large/04/040210261.jpg
The only difference on that one is the channle isnt tappered.