Wait inside Heavy Containment's Nuclear Warhead room. Make sure the latch on the table is activated and pointing upwards so that someone may activate the nuke on the surface, or activate the nuke yourself if you have enough time. Once the nuke alarm has begun, wait until the red clock on the table is at 15 seconds, then quickly flip the latch to face downwards and press the large red button next to it.
Once the clock goes below 10 seconds you can no longer deactivate the nuke, so ensure you do this between 10 and 15 seconds on the timer.
An easy way to do this is when you're spawned in as a solo SCP. Do not kill any D-class or scientists, but simply wait inside the nuke room, leaving the nuke latch enabled. Eventually, someone will likely activate the nuke on the surface, as you did not intrude on any D-class or Scientists upgrading their cards. When this happens, guard the Heavy Containment nuke room and only deactivate it when it's below 15 seconds. If you spawn in as SCP 106, you can hide inside the armory within the Warhead Room if need be.