Some spouses will be taken completely by surprise and shocked. Others will have been considering divorce themselves for some time. Others will be at various points across the spectrum in between these two extremes.
Your spouse’s reaction will depend on where on the spectrum he or she is currently “at”.
You should wait for a block of quiet, private time, and ensure there will be no interruptions.
Don’t assign blame. Just explain that your marriage is not working any more, but that you want to resolve things amicably and fairly.
After you reveal your decision, do not let yourself be baited into a debate about it, and don’t attempt to defend your decision.
And don’t dive into negotiations. It is too soon. Your spouse needs time to accept and adjust.