You are smart to plan. You can pay online at the time of your conversion. This will be an estimated payment for the 4th Quarter. You can also mail in a 4th Quarter 1040-ES voucher to the IRS by January 15, 2019. The same would apply to your California return.
As you already know, when it comes to making estimated payments all that matters is that you have enough paid in for the quarter. It can be from withholding through a pension or other income sources. You could also ask the IRA custodian to withhold the tax. However, this is generally not advised as this will reduce the amount of the tax-exempt income now working for you in your Roth.
