I started the Diploma through the Permaculture Association in 2008 and finally accredited at the London Permaculture Festival in July 2013. The diploma process is Action Learning personified. You are assigned a tutor who is more a mentor really. With them you decide on a tentative Action Learning Pathway - what you think your required 10 projects may cover, when you might do them and what additional courses or learning you may need. Then this goes out the window once you start! You learn through practically applying all the theory you have from the Permaculture Design Certificate, which means by trying things out and making mistakes.
For loads of great stuff on what's involved in a Permaculture Diploma or how to get on an Introduction or Full Design Course go to the Permaculture Association website.
You can download some of my projects as word documents for free, I offer this to help encourage others to take the diploma and then share their knowledge and experience.