These figure of 8 clips are used to fit table or bench tops to the supporting rail. Cut a recess the thickness of the clip (2mm) into the top of the rail (easily done with a router). Then screw the clip into the top of the rail and then screw the other hole in the clip to the under side of the table or bench top.
Please note that while this is a simple and easy solution, it doesn't allow for any timber movement over time and may split the two work pieces being joined. To take timber movement into account, look at using the Z-table clips which allow for natural timber movement over time.