Lisa at U-Handblog has a tutorial showing us how to add a purse tongue lock to a handbag.
Lisa writes:

Purse tongue locks are the sprung locks that you often find on those lovely leather document cases or satchels. They look so nice (and super-duper professional) on bags and clutches and as a design feature they really do add interest to your bags. I have just managed to get my mitts on some pretty looking and professional quality Silver Purse Tongue Locks for my shop so I thought this was a good time to post a tutorial on how to apply them to your bags and purses. They are simple to use and they give brilliant results.