Tegmark, Samotsvety, and  Metaculus provide probability estimations for the use of nuclear weapons in Ukraine, and the start of the full-scale nuclear wars (kaboom  and KABOOM in the terminology of Tegmark). kaboom will certainly be a warning sign before KABOOM, so it makes sense to think beforehand about the action plan in the case of kaboom. 

Individual plans

I am aware of two potential strategies of survival during the nuclear war. First,  you can hide in a shelter, and for preparation I recommend to read "Nuclear war survival skills" . Second, you can try to be in a place minimally affected by nuclear war. In my opinion, the second strategy is better in a long run, since the life in the bombed territory will be most likely far worse than on the untouched one, even if today this untouched territory is a relatively poor country. Thus, as soon as the warning event (little kaboom, using nuclear weapons against Ukraine) happens, I would move for some period of time to one of the countries that are unlikely to be bombed. Here is my quick and dirty estimate of which countries are potentially good for this purpose. Suppose a full scale Russia-NATO war. Then:
-Obviously Russia and NATO countries are not good (a couple of small NATO countries like Montenegro may survive but it is a risky decision). Potentially NATO alliances like Israel are also dangerous, but so far Russian-Israel relationships are quite good so I would consider it much less likely. 
-Tegmark argues that China will be struck by US so that it would not become the dominant power after the war, referring to this declassified official US strategy. Thus, China seems to be very risky option as well.
-If China indeed  is struck by USA, Australia is not safe because of AUKUS. If, moreover, China uses the same strategy of getting rid of potential dominators, India could be under attack.  

Thus, I think that countries and regions that are not likely to be struck include:
Latin America
Africa
New Zealand and Pacific (Australia has some risk)
Asia except: China, maybe India and Pakistan, and maybe countries of CSTO 
Non-NATO European countries (the safest bet would be Switzerland). 

Also, a remote part of an attacked country may still work if there is no military base there. 

Now, of course, there's the question of fallout and nuclear winter. Will this country or region be capable to survive the consequences? This is a way more complicated question, due to many parameters. Take, for example, Switzerland. On the one hand, it is surrounded by NATO countries, so fallout will be everywhere. On the other, big parts of it are in the mountains, which will likely decrease the problem. On the one hand, all economical connections with Europe will be destroyed, which potentially can cause hunger. On the other, the Swiss government spent a lot for preparation to the fallout during the Cold War, and so may be still well prepared. Thus, each country requires attentive study. And one of the important parameters in this study is how well the country you choose fits you. 


It is good to have a list of few options. All tickets to the New Zealand were sold in half an hour after kaboom? Well get tickets to option 2, 3, or 4. Sold out tickets are not exaggeration, by the way. For example, within minutes after the "partial" mobilisation in Russia was declared, all tickets to the closest days to that event were sold out.   
 

Finally, the ability to fly away with your loved ones immediately as a small kaboom happens is crucial. So it needs to be discussed beforehand, all documents need to be valid, the job should be ok with your sudden disappearance for a couple of weeks, etc.

Plans for organisations and movements

While before we discussed what to do on a personal level, it is important also to think how to ensure survival and flourishing of EA (or rationalist movement, or any other movement you support) after the nuclear war. Here one may consider the following questions:
 

-Is EA already present in the safe countries? If so, how well? Would it be able to function and continue most of its ideas without destroyed EA hubs?
-After the war, how can different EA chapters communicate?
-Is it possible to organize a quick evacuation of EA members from hubs to safe countries in case of kaboom? To make sure on the organisational level, that everyone is well-informed and prepared?
-Can EA in safe countries ( with the help of global EA) prepare beforehand for the potential consequences of fallout and nuclear winter?

Timelines 

Looking at the current situation, I think the nuclear strike to Ukraine is highly unlikely prior to the effects of mobilisation (that is, winter or spring). Before that Putin was always going to new escalation when the old one did not bring results, so I would predict no next escalation before the results of the mobilization. This gives us at least a couple of months, which is definitely enough time to discuss your plans and likely enough to plan something with your organization.