Normally with the 5V version of this motor the common wire is red.
But you can ask the motor itself with a multimeter. Use resistance range
and measure the resistances between pairs of wires. The common wire is the
one which measures the smaller resistance between it and other wires.
Common wire goes to +12V, as does the COM pin of the ULN2003 (this
is vital).
As for the sequence of connecting the other wires you can experiment, 8
ways of wiring it up will work (4 for each direction), 16 ways will fail.
However you can fix one wire arbitrarily as the first phase, then there are
only 6 possibilities, 2 of which work and 4 don't.
Just remember never to disconnect a stepper motor's wires while it is powered up,
this is likely to fry the driver. (It certainly will with big steppers).