Where is the problem ?

1. You must know the maximum current.
2. You can choose your shunt (don't forget the heat!!!!)
3. You use a OPAMP as an amplifier for the voltage to maximum 5V for maximum current trough the shunt. I use an LT1490 (rail-to-rail) for 5V-supply and good quality.
4. Measure the voltage !
(I use the lm385-2.5 as a reference. I measure the voltage of one port an the voltage of a port with the lm385 on it to calculate the correct voltage, because the supply-voltage may be incorrect or spread widely. The normal 7805 is worse, I use a 78M05CDT instead.)