To accomplish this, you will need a DPDT relay. Use one of the poles for your holding circuit, and the other one to switch your load. You will also need 2 momentary switches: One normally open, and the other normally closed.
Wire the coil of the relay through one of the poles of the relay, with a normally open switch across it in parallel. Pressing the switch will activate the relay, and then it will hold itself on after you release the switch.
Wire a normally closed momentary switch in series with your holding circuit. This will be your "off" switch. Pressing it while the relay is energized will break the control circuit, causing it to turn off.
Wire the coil of the relay through one of the poles of the relay, with a normally open switch across it in parallel. Pressing the switch will activate the relay, and then it will hold itself on after you release the switch.
Wire a normally closed momentary switch in series with your holding circuit. This will be your "off" switch. Pressing it while the relay is energized will break the control circuit, causing it to turn off.