Andrew Whitaker put together this 3-part video which walks you through the process of developing custom exploits beginning with "fuzzing" and ending with an automated module for the Metasploit Framework.

Andrew Whitaker is a nationally recognized expert on computer hacking, an author of best selling networking and security books, and an award winning technical trainer. His books include Penetration Testing and Network Defense (Cisco Press) and Chained Exploits: Advanced Hacking Attacks From Start to Finish (Addison-Wesley - co-authored with Keatron Evans who is also featured on this site). He is a frequent conference speaker and has given talks at DefCon, TakeDownCon, ChicagoCon, and SecurePhilly. He holds over 20 certifications and is the recipient of both the EC-Council Instructor of Excellence award and the EC-Council Instructor of the Year award.

Be sure to check out Andrew's website for additional information:

http://hackamuffin.com/Hackamuffin/