They come with instructions, and it is probably easier to understand when you see the diagram that comes with it - but I will try.
The tools you will need are a saw suitable for cutting the plastic downpipe (a fine blade hacksaw is good) and a drill capable of cutting a circular hole in the butt (if it doesn't already have one - some do). You will also need a Stanley-type knife to cut out the correct template for the water pipe shape.
It isn't difficult but the critical thing is getting the height of the cut in the downpipe in precisely the right place (vertically). The instructions make much of this - but it isn't too hard. The height of this cut is important because it determines the point at which water ceases to fill the butt and starts to transfer excess water back down the downpipe again to the soakaway. If the cut is made too high, water keeps flowing into the butt (which then keeps overflowing); get it too low and no water will transfer into the butt at all.
The rest of the device is push-fit together.