It really only works if you have the "waterfall" type filter.
What you do is:
-measure the width of your filter and cut the bottle accordingly
-cut a crescent shape in one side of the bottle (to wrap around the bottom of your filter
-cut notches on the "top" part of your bottle so it holds onto the filter better
-if any part of your bottle will be within reach of your fish make sure you buy either duct tape or electrical tape and tape the cut edges so they are soft.
here are a few pics of my set up...
I hope this helps.. sorry my pics aren't so good.. my camera is kinda crappy.
You can also put pantyhose over the intake to slow the flow down.