Ok, either use the Activator app or go open the Settings app and go to the Jailbreak tweak/app section and chose "Activator.
There you can chose where your new assignment should work, you can chose Everywhere, homescreen, Lockscreen and in Apps.
You can also create menus and more. Just go to "Everywhere" and there select a gesture.
Search in the list for "Pinch", it's one of the springboard actions.
The it will ask you what should happen, then select "Simulate Sleep button" or "lock device" in system actions.
Leave the app and try it on your homescreen. It should lock your device now.
If you still need help, then I can also create a step-by-step picture or video tutorial