Next time I'll be sure to have enough cash on hand to pay all the taxes separately before performing my roth conversion.
If you usually get a tax refund, or you have non-refundable credits that you don't use, then you don't even need a check--just tell them not to withhold.
Doing so, and significantly underestimating the taxes you will owe, could result in penalties come April 15, because you under-withheld. But if you know your situation well enough, you can breeze through this step.
I am FIRE'd, living off long-term capital gains from my taxable account, with ACA subsidies, and 1 kid. I am doing Roth rollovers at the end of the year to balance / consume my tax credits, since most of my income is taxed at 0%. I buy tax software as soon as it's available (Nov 4 this year) and do a pro-forma return. Then I roll over the amount that leaves me at or close to a zero balance.