If I understand your question correctly you are asking how, for example, to insert a document in a "category" collection that contains a "dep_id" field containing the _id of a department such as "clothing"?
Here is an example of how you would do that using the mongo shell:
clothing = db.department.findOne({dep_name: "clothing"})
db.category.insert({dep_id: clothing._id, cat_name: "women"})
Does this answer your question?
Bruce