If you are NOT drawing Social Security:
Once you apply for Medicare and are approved you will be sent a bill for 3 months advanced premium ($445.50) due before your Medicare even starts. You can create an account at mymedicare.gov where you can pay that initial bill and you may want to sign up for Medicare Easy Pay. If you sign up for Easy Pay, Medicare will bill all future premiums monthly around the 20th of each month. If you do NOT sign up for Easy Pay, Medicare will bill you quarterly (every 3 months) via paper bill.
To prevent falling behind in payments, which will result in your losing Medicare and any other plan you signed up for (MAPD or Supplement), its best to sign up for Easy Pay after you pay the initial 3 months advance.
If you ARE drawing Social Security and elected to have your premium withdrawn from your Social Security check, they will bill your premium monthly ($148.50). There will be no 3 month advance pay.
Keep in mind that if you owe IRMAA due to having a higher income or have any late enrollment penalties, you will also be billed that adjusted rate as well.