There are various ways to freeze a document (and any objects it contains) in the current branch.
Right-click a document, folder, or root node and select the Freeze command. This command is available only if you have the Freeze versions right and the appropriate permissions on the documents.
Use the Freeze tool on the property sheet of a:
Before or after check in using the options in the Check In Document and Check In Project windows (see Checking in Changes to Documents Stored in the Repository)
You must have the Write or Full permission to freeze a document version.
This option specifies that the Freeze after check in option in the Check In window is selected by default.