Once your working copy is updated to a branch, just use CVS as you normally would.
Only difference you will see is that revision numbers are 4 sets of digits instead of two. (Branch from revision 1.4 yields branch revisions of 1.4.2.1) Example:
$ cvs status hello.py =================================================================== File: hello.py Status: Up-to-date Working revision: 1.4.2.1 Tue Jun 12 14:30:36 2001 Repository revision: 1.4.2.1 /home/cvsroot/hello/hello.py,v Sticky Tag: Release-1-0-branch (branch: 1.4.2) Sticky Date: (none) Sticky Options: (none) $
You can switch between branch and main trunk simply by using the update command on your working copy.