The Medicare website provides you with guidance on paying your Medicare Part B (out-patient and doctor visit coverage) and notes that if you get:

• Social Security,
• Railroad Retirement Board (RRB) benefits, or
• Civil Service benefits, 

then your Medicare Part B (Medical Insurance) premium will get automatically deducted from your benefit payment (for instance, your Social Security check).

However, if you do not receive Social Security, RRB, or Civil Service benefit payments - and you sign up for Medicare Part B, Medicare will send you a bill for your coverage - this is called a “Medicare Premium Bill” (and is designated as form CMS-500).

Please note, if you buy Medicare Part A (Hospital Insurance) coverage or you owe the Medicare Part D income-related monthly adjustment amount (IRMAA), you will also receive a monthly bill for your premium (or additional IRMAA payment).

Medicare provides four ways to pay these monthly bills:

1.  Pay directly from your bank account through your bank's online bill payment service. Contact your bank or go to their website to set up this service.

2. You can sign up for Medicare Easy Pay, a free service that automatically deducts your premium payments from your savings or checking account each month.  Medicare will deduct your premium from your bank account, usually on the 20th of the month.

3. You can pay by check or money order. Mail your Medicare premium payments to:

Medicare Premium Collection Center
P.O. Box 790355
St. Louis, MO 63179-0355

4. You can pay by credit card. Complete the bottom portion of the payment coupon on your Medicare bill, and sign it. You will need to provide the account information as it appears on your card and the expiration date. Most credit cards today only have the month and year in expiration date field. If your credit card only has a month and year in the expiration date, fill in the month and year on the payment coupon and leave the day field blank.  You can then mail your payment to the Medicare Premium Collection Center address above.

If you get a monthly premium bill from the Railroad Retirement Board (RRB):
Mail your monthly premium payments to:

RRB, Medicare Premium Payments
P.O. Box 979024
St. Louis, MO 63197-9000

If you are a Civil Service retiree and NOT entitled to Social Security:
You may have your premiums deducted from your Civil Service annuity. To do this, send an email to OPMMailbox@cms.hhs.gov.

If you have limited income and financial resources:
Your state may help you pay for Medicare Part A, and/or Medicare Part B. You may also qualify for Extra Help to pay for your Medicare Part D prescription drug coverage.

Sources include:

"Online Bill Payment for Medicare Premiums" https://blog.ssa.gov/online-bill-payment-for-medicare-premiums/

And from the Social Security Administration's online handbook:

Section 127.5 How are Medicare premiums paid?

Medicare Part B premiums must be deducted from Social Security benefits if the monthly benefit covers the deduction. If the monthly benefit does not cover the full deduction, the beneficiary is billed. Beneficiaries may elect deduction of Medicare Part C (Medicare Advantage) from their Social Security benefit. Some Medicare Advantage plans include a reduction in the Part B premium. Social Security takes that reduction into account, as soon as we are notified of the reduction by CMS. Beneficiaries may also elect deduction of their prescription drug plan premium from their Social Security benefit. (https://www.ssa.gov/OP_Home/handbook/handbook.01/handbook-0127.html)