Congratulations, you're now a business. You'll be filing Schedule C and Schedule SE along with your other tax forms. You'll be eligible for the the Qualified Business Income Deduction unless you have high income from other sources (over $157k if single, I think. More if married).
You'll also qualify for a business retirement plan, or this income would qualify as earned income for IRA contributions for both you and a spouse. This would be a good option if you want to "transfer" money in a taxable account to Roth.
Self employment taxes are how you pay both the employer and employee half of social security and medicare taxes. These are calculated on on Schedule SE. They will be about 14.12955% of your net income from the business. Add this to your marginal income tax rate and that's how much you should plan to pay in taxes.
You're responsible for paying taxes throughout the year. You may already be familiar with this if you pay estimated taxes on your retirement income, but you need to pay taxes throughout the year in 4 equal installments (the easy option) or as the income is earned (more complicated option). Alternatively, if you're over 59.5 and you make IRA withdrawals you could have money withheld from those withdrawals (bad idea under 59.5 since you'd pay a penalty on the withholding, unless you're on a SEPP withdrawal plan). Withholding, unlike estimated payments, is assumed to be timed properly regardless of when it's actually paid. Here's more information, including the safe harbors under which you won't owe a penalty even if you don't pay all of your tax liability for the year (see under the heading "Penalty for Underpayment of Estimated Tax"):
https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxesMake sure you reduce your business income with any expenses you feel you can justify. If you have a space in your home that you can dedicate to "regular and exclusive use" by your business you can take a home office deduction. If you have a legitimate home office and you make sure you do some business there at both the beginning and end of your workday you could deduct for business mileage for any driving you do for business. This is because the first and last place you travel to is considered commuting and only travel between work sites is deductible. But if you do some work in your home office before and after driving to/from other work sites, then your commute is walking to your home office and the drive is business miles. You might be able to come up with other business expenses, but those are the big "freebies" that don't cost you extra money that I can think of.
You might be able to deduct the cost of health insurance against your business income. I'm not familiar with this as we get health insurance through my wife's job.
