This guide will assume you are using IntelliJ IDEA as your main Java IDE. At Minespree, we follow the IDEA module pattern directory schema, which means every Java project on GitLab is a module on a project you create. This way, you don't have to keep opening and closing projects, which takes a while when you have more than 10 modules.
Go to the GitHub project page, copy the Git clone (the git SSH protocol is preferred), open a terminal, cd to your project root directory and run the following:
git clone git@github.com:minespree/<Project Name>.git Directory_Name/Name your modules appropiately, as changing them in the future is a pain. Once you have cloned it, go to IntelliJ, click on File -> New -> Module from Existing Sources...
Finally, select the Directory_Name/ that you just using git clone, click OK and then just spam the Next button until you're done.
As you probably have cloned the SpreeGot project, IntelliJ is assuming your source code packages will look the same as the produced output by running mvn install. As Spigot or the Feather project perform package relocations by using the maven-shade-plugin, IntelliJ IDEA doesn't know about the final package names as the relocation stage is run on the install phase, whereas IntelliJ IDEA only cares about the compile phase.
The best way to fix this it to ignore the Maven pom.xml by clicking on the Spigot-Server package -> Maven -> Ignore module(s):
IntelliJ will ask you if you want to remove the module from the project, click on No.
There's an open issue on YouTrack about this: https://youtrack.jetbrains.com/issue/IDEA-126596