You can mount the smaller rail sections directly under the gas block as long as you are using a Troy lo-pro gas block, or similar gas block. Gas blocks that use side set screws or some other gas blocks that are more bulky on the lower portion may cause interference.
The easiest way to install these is to slide them on the inside of the rail to the desired locations, and then slide a screwdriver or allen key through two of the holes to hold the backer plate in place while securing the screws. If you purchased this from Troy it should have come with instructions. You can download them from our website under the "rifle technology" area as well.