Very short version: Go to the SSA website, obtain Form 44 which is a PDF. Fill it out and snail-mail (yep) it to your local SSA office. It’s pretty simple.
Form 44 is designed for exactly that situation, the transition year(s) when your taxable income craters upon retirement. You tell them what you expect to have for taxable income in 2027 and they just take your word for it. But … if you get it wrong enough they’ll (eventually) bill you for any unpaid IRMAA that later tax returns show you should have paid waay back when in 2027.
Longer versions ...
You can read about it upthread here: