If this is the tutorial by Jono Bacon on Youtube, launch glade with
quickly design and add a button to the toolbar similar to the refresh button that Jono adds ~12:30. Call it backbutton and create a handler for it when clicked and then sketch a function that will pull the previous URL that was passed (just create a list and append successful URLs to it, then pop it when needed). If you want to preprocess a URL entered in the entrybox, add code in the handler to do it. Here's some sample code:
#Code for other initializing....
....whatever goes here...
#store previous location in history
url = widget.get_text()
#add http:// if not present in the url
if 'http://' not in url:
#Only get the last url if there's something to get
#Use pop to remove the last entry from the list
This code is not very sophisticated, but should give you the idea.
As Timo has pointed out, WebKitGTK has a number of great functions and features including a go_back() method (If you read the manual). Here's a callback function which uses it:
def on_backbutton_activate(self, widget):