If you want to make some small changes in a vendor package and you don't want to fork it on GitHub, here is the instruction what you should do:
1. For example, you know the name of the package, it is, for example, leroy-merlin-br/larasniffer. Search it's repository in GitHub.
2. Create a directory called 'workbench' under the Laravel's root directory.
3. Create a dir with vendor name, for example, leroy-merlin-br, open it: ...