I have some experience with a few completed game projects and I am personally alergic to too much pre-code design, and I am repulsed by things like UML. If I were to make a UML diagram I would probably spend all my time admiring and optimizing the diagram and I wouldn't start coding at all.
I think the best option is to have a good and clear view of what your project should be, in your head.You must also have enough experience to know what you can do and what you can't and restrict yourself to the former.
Before starting, divide your project in intermediate steps corresponding to what you can identify as the difficult parts.and write down this roadmap somewhere (I use notepad). Try to think modular and if possible make little "bricks" of program that you can use elsewhere and to help you build the whole.
While coding you'll probably think of things to add and you'll notice bugs to resolve.As soon as you think of it you should write it down (for future processing) and come back to the task you are dealing with.
You must make a hierarchy for the importance of the different tasks, solve the urgent problems and code structural backbone in first , at the end you'll take care of bell and whistles.
Anyway I am working alone and if I was part of a team things would be different and I'd probably need diagrams to explaion things to people.
Now to come back to your particular case, I think your problem is a lack of experience with java because there isn't so much difference between java and VB , if you could do it in VB there's no reason for not being able to do it in java.
you are probably confused by the different structure of the Java code,you should take a look at some simple algorythms and examples because Java is generally a good language to let your ideas flowing.