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If you were hoping for a list of 20 steps that you can apply without thinking, it won't do you any good. Fortunately, the Maven plugin is perfectly able to rebuild the projects correctly starting from the bare POM, so often the only thing you have to do is scrap an existing incorrect Eclipse project configuration and have a new one rebuilt.
The End Result
The first step is to describe the target result. Then a sequence of "repair" steps will demonstrate how to clear errors and rebuild the projects to achieve this desired end result.
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If you get this structure right (one big "cas3" directory/project containing all the source files but regarded by Eclipse as having no Java source directories to compile, then one project per subdirectory that compiles and builds the source for that one module) then the Eclipse Maven plugin is prepared to get the rest of the Eclipse project configuration right (or close to right) if you take steps to clean up the residue of any misconfiguration left behind by the original JA-SIG files.
Check Out
Display the SVN Repository Exploiring Perspective (Window - Open Perspective - Other - SVN Repository Exploring). If the JA-SIG repository has not already been added, right lick in the SVN Repositories view and choose New ... Repository Location. The URL is https:source.jasign.org and that becomes the name of the Repository. Expand this repository as a tree, expand "cas3". Now decide if you want to work on the unreleased next version of the server ("trunk") or one of the numbered previous releases ("tags"). Expanding "trunk" or one of the "tags" should show a pom.xml file and subdirectories including "cas-server-core". Choose the directory ("trunk" or "cas-server-3.4.2") right click, and choose Check Out...
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