The best way is to get a job with a company that has JDE or with a consulting firm that does JDE contracts. There are so many pieces to get familiar with in order to develop the "JDE" way that unless you are incredibly dedicated, have lots of free time, and have access to JDE software, you won't be able to make much progress. I have known one or two people that can just read a technical manual, understand, and apply it but most of us can't, hence the need for on-going interaction with the product. Additionally if you don't have a job dealing with JDE, why learn it? You might be banking on the fact that you could get a job with a company that had JDE if you already know it but if you haven't dealt with it in a production environment I would think most companies would not take "I learned it at home" as enough experience.
If you are still determined to move ahead, get the World or One World Developers guide shown on the JDEList site.
St Louis MO
Opinions expressed are not necessarily those of my employer