You will need two M8 screews and two 608 bearings. The guide is designed to sit loosely on the axle so it can move when it needs to.
This means, the filament needs to be positioned lower than the guide, or it may come off. (See image of my setup in the Gallery)
I have used two filament guides before, one that was fully printed and which got eaten away by the rougher filaments, another one - with one bearing which tended to make quite some noise when the filament switched from one side to another.
Open Scad file is attached so you can adjust the bottom hole for a tighter fit.