If you have Firefox you will need to install Greasemonkey
Go Here and click "Add To Firefox" it will download you may be asked if you want to install it say yes, you may also need to restart firefox now.
Now click on the link to the script you want to add, there are two around at the moment that add OS Maps and the other Satellite view, you will get a pop-up box asking if you want to install this script click Install, repeat for the other script then you may or may not need to restart Firefox.
Since chrome now supports scripts on its own you should just be able to click on the links to the scripts and click Install then click ok in the warning you get in the bottom left then click Install again.
the attached pics show how it looks in both when you have done it right.