Step #1 Label the Circuit … all nodes. One of the nodes…node A, is chosen as the reference node. It can be thought of as a circuit ground, which is at zero voltage or ground potential.
Node B and node D are already known to be at the potential of the source voltages. The voltage at node C the voltage VC is unknown.
Let’s assume that VC > the voltage at node B and VC > the voltage at node D when all three currents are drawn arbitrarily… remember, these directions are arbitrary and may change depending on the outcome of the mathematics.
The direction of I1, I2, and I3 is assumed to be emanating from node C, and toward the reference node A.