To convert a galvanometer into a voltmeter one should connect a : -

A voltmeter is connected in parallel across the element whose potential difference is to be measured, and must draw negligible current. So it must have a very high resistance.
A galvanometer is converted into a voltmeter by connecting a high resistance R in series with it: $R = \dfrac{V}{I_g} - G$.
(A low resistance in parallel, a shunt, converts it into an ammeter.)
So one should connect a high resistance in series with the galvanometer.