It depends if you have self employed business income reported on Schedule C. If yes, you will enter the Medicare payments there as a deduction from your profits. Click the BUSINESS tab, click "I'll choose what I want to work on". Look for "Business Income and Expenses", Click the start or update button. Click "Edit". Look for the section "Business Expenses" and choose the sub-section called "Other Common Business Expenses". Look for "Insurance payments". Click start or update and there you can enter the Medicare parts B and D.
If you do not have separate self employed income, click the PERSONAL tab, then "I'll choose what I want to work on". Scroll down to Retirement Plans and Social Security. Click the start or update button for Social Security. The next page will have boxes to enter your Medicare payments.