see, it is a very simple experiment....

 

first set up the circuit as given in ur book..

 

first introduce only the resistance box of desired resistance  ( high resistance one) into the circuit and  count the number of divisions as indicated by the galvanometer.......see, preferably choose such a resistance...so that u get even number reading from the galvanometer(this even number is just for our simplicity so that we r accurate when we r finding half deflection.)

 

then repeat the same thing now.....but  by introducing the shunt resistance in parallel circuit. take out that value of resistance from R.B that u get exactly half the no .  of divisions in galvanometer in comparison to the previous one.

 

note all the readings.....put it in formula given... to find the resistance of galvanometer..u r done with the experiment

 

 

u will also find that resistance of galvanometer is very very close to resistance of shunt ...  (sometimes even same)....
this is bcoz when the galvanometer is showing half deflection it means current has reduced to half in the main circuit and the other half current is flowing through the parallel circuit containg shunt.  and when currents are same across galvanometer and shunt.......it means their resistance has to b same (since in parallel their voltages r same)

hope u understood this...