Depends a little on the jurisdiction for details but basically I think the key is to do it swiftly, directly, unreversable and on both sides: benefit cutting + cut financing.

So for the US I'd instantly abolish the payroll taxes that fund Social Security, widely communicate that there won't be any government funds for retirees going forward and that you're responsible for your own savings.

Then: Income/housing-wealth test all current retirees, and cut off benefits for anybody above like median income or whatever (so half? One-third of claimants?) and finally use the remaining ~3tn or so to keep paying for existing, non-wealthy retirees until it runs out (2-5 years).

That way you a) instantly creates savings for current workers and income-earners — who are the friends, neighbors, children of current retirees — and b) give time to poor(ish) peeps who deserve some support, stretching the remaining funds for some reasonable time. (Get your affairs in order, have your kids help you out).

...and obvs, abolish the Fed and Congress and everything else that makes economic life for the elderly worse