As you can see the entrance of the sluice box is slightly higher than the rear.  You can also see the small dam that I made using stones laying in the creek bed.  If the water is not flowing fast enough, a small dam can help.  It raises the water level, water being water will take the path of least resistance.  Across the sluice box.  The flare forces the water to run quicker through the box.

This is the same setup, just a head on view.  Point the box aiming with the stream of water.  If it is diagonal to the water flow the concentrates tend to build up on one side.  You want the material to flow evenly across the riffles.  It can separate the material in a very effective manner if it is allowed to handle the material evenly.

Same setup as above.  Notice the > shape extending from the corners of the carpet to the middle of the third riffle.  This can act as an indicator to how fast the water is flowing, how evenly it is flowing and if the box is positioned wrong. 

This is a close up of the bed of the sluice box.  The flare is to the left the exit is to the right.  You can see the riffles.  Look closely, you can see the small ridges, or riffles in the carpet itself.  These are VERY effective in separating out the fine gold and black sands.  Offering them a secure place to hide.  Most of the sands and gold are caught by the time it reaches the third riffle.