Can flight attendants make their own labels for use when working, and is there a regulation that prohibits them from sticking one onto a bin or cart?
Labels temporarily affixed to a bin or cart does not make it a placard. It’s a label and it does not require any regulatory approval. Placards are markings, data, and other required informational markings that must be installed on a plane as required by regulation, such as bin weight restrictions, exits, last date of inspection (as required), no smoking, emergency equipment placards, etc. There is plenty of supporting information found within a document called the Cabin Safety Subject Index, under the topic: Placards. Additionally, all aircraft operators have a maintenance manual Chapter 11, which is a chapter dedicated to aircraft placards, which are required to be present in specific locations.
As for labels that flight attendants wish to make and use in their galley, whether or not an airline’s management will allow flight attendants to temporarily attach personally made labels to bins, carts, or stowage areas, that is something an airline can permit or restrict.