The Salifert test is not a particularly useful test for reef tanks as it is too dificult to distinguish the colors under 0.1 ppm.
In order to read the test you should take off the cap, place the vial on the chart, and look down from above under daylight lighting, preferably outdoors.
If you intend keeping sensitive coral, you will need either the Red Sea Pro Test, or preferably the Hanna ULR Phosphate checker.
To slowly reduce phospates (if necessary) you could use an aluminium based remover such as Phosguard in a bag in your filter/sump, or GFO in a reactor in the sump.