Jonny,
Basically you use a runrig and attach a subfloat to the mainline, at the other end of the sub float you tie on your hooklength.
Once cast out pay off the line exactly the same as using a marker float until the float hits the surface. You can now either surface fish, or by winding back down say 5 foot of line, and you would be fishing 2 feet below the surface.
By using this method you can fish at all depths from the length of your hooklength off the bottom right the way to the surface.
Hope you can follow that lot!