1. Download this repo as a ZIP file from GitHub.
2. Unzip the file and you should have a folder named chrome-extension-remove-reddit-ads-main.
3. In Chrome go to the extensions page (chrome:https://extensions).
4. Enable Developer Mode.
5. Drag the chrome-extension-remove-reddit-ads-main folder anywhere on the page to import it (do not delete the folder afterwards).

Reddit is making impossible to block their ad with a CSS selector.