ln is the Linux command for creating a link to a file or folder.
To create a relative link:
ln -r --symbolic ./<existing file> <name for link>
Relative links will still work even if the folder containing both files (the original and the link) is moved or copied.
I pretty much always get confused as to which is the link and which is the target, so here's an example. You're inside «» and you want to create a link called "lib" to the "lib" folder at « »:
ln -r --symbolic ../../lib/ ./lib
..or, in other words:
ln -r --symbolic <actual file> <link-to-file>