general points:
first of all it's very important to have it temperated. for that you need
a thermostat, which you connect to the refractometer. (it surely has
two connections for that). refraction indices are highly temperature
dependant and are usually given for 20°C.
always keep the glass clean (eg with alcohol), take care not to
scratch it!
how to use:
* open the thing (the moving part you talk about), place a drop of your
substance on it and close it.
* look through the "tube". now turn the big knob and somewhere you should
see a sharp border between a bright and a dark region. adjust this
border to be exactly at the center.
* read the number. this is your refraction index.
* be amazed at how impure your product is
* open the thing and clean it with a tissue and some alcohol (bottom and top).
close it. done.