You’ll need a swimsuit, robe, bottle of water, and wool hat to feel like a regular at the Royal Palace (614 Sheepshead Bay Rd., nr. W. 8th St.; 877-819-2284), a sprawling and endearingly gaudy sauna complex. Pay the $40 day fee and leave your valuables with the cashier; you’ll get a locker, some towels, and a pair of flip-flops. Loosen up in the whirlpool, then join the pink-skinned, mostly young Brightonites lying on marble slabs in the scalding Turkish steam room. Try to stay five minutes. Move to the Roman sauna, dousing your head with buckets of icy water every few minutes. Next, with your hat on, progress through the stone-heated Russian saunas of varying temps (No. 5 is unbearable), with dunks into plunge pools to cool your core and drop your heart rate. Counterintuitive though it seems, a hat regulates head temperature, which in turn regulates body temperature, so you can boil and freeze longer.