You need a shunt to use them as ammeters.
They will work just fine, but you will need to know a few things first.
1 - Voltage across the meter to hit full scale. This is in the mV range; 50-100 is typical.
2 - The maximum current you need to measure.
3 - Then calculate the resistance of the shunt so that the voltage drop across the shunt and the full scale meter voltage line up.
4 - Mock it up to test it.
The higher the current, the lower the value of the shunt. Above several amps, you can get away with using just a length of solid copper wire (like the ground from some Romex).
To connect the meter to the shunt, connect the negative side of the meter to one end of the copper wire. The positive will go somewhere along the length of the wire toward the other end.
To calibrate it:
Tie a calibrated meter in series with your homemade meter and slide the positive meter lead along the copper wire. When the meter matches the range you intend to measure, solder it at that spot on the wire.
To achieve lower currents at full scale, the positive lead from the meter will go further away from the negative.
If the wire ends up being of any significant length, you can fold it in a zig-zag pattern and glue it to a substrate to make it more compact. Just don't let it short out on itself, or you'll throw the calibration.
Voila! Mark the scale appropriately and put it in a case!
Steve
EDIT - In case it's not immediately clear, the terminals to your new ammeter are on the outside of the copper. Like so:
(-) ------X---------------------------------X----- (+)
The X's represent the connections to the meter movement.