Make a Latching Relay using SPDT Relays
Both of the two solutions below use one button for turning on, and a second button for turning off.  The difference between the two, is thr first uses a NC button for turning off, which may not be available.  The second solution uses NO buttons for both actions.

In both cases, when the first button is pressed, the relay activates, and the contact is closed holding the relay in the closed position until the circuit is interrupted.



Doing both the on and off actions with one button is considerably more complicated.