Seen this elsewhere and thought it might be of use, don't think I would chance doing it myself but seems fairly straight forward

http://bmwz3diys.weebly.com/coil-on-plug-conversion-bmw-z3-19-m44.html

Original post on Bimmerforum (ohhh sorry B**********)

http://forums.bimmerforums.com/forum/showthread.php?1103482-DIY-Coil-on-plug-conversion-w-electrical-diagram