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.
If there's anything you're still unsure about, please contact the Studielijn:
071 527 11 11