The student website describes how Leiden students should apply for minors and explains the relevant criteria.
Information about minors offered by Leiden University are listed in the e-Prospectus.
You may find the answer to your question in the FAQ for students.
You can read more about the joint minors on the website of the Leiden-Delft-Erasmus Universities.
If there's anything you're still unsure about, please contact the Studielijn:
071 527 11 11