How is a voltmeter connected in a circuit?

If the work done to take $3 \,C$ electric charge from one point to another point is $15 \,J$,what will be the potential difference between these two points (in $,V$)?

How is a voltmeter connected in the circuit to measure the potential difference between two points?

$A$ battery is a source of maintaining a constant potential difference across two ends of a conductor.

How much work is done in moving a charge of $2 \, C$ across two points having a potential difference of $12 \, V$ (in $, J$)?

What is meant by saying that the potential difference between two points is $1$ $V$?