Summarize the following:
Click the Safari app icon, which resembles a blue compass, in your Mac's Dock. Go to https://adblockplus.org/en/download in Safari. It's a link under the "BLOCK ADS ON ANY DESKTOP BROWSER" heading on the left side of the page. Click the "Downloads" arrow in the upper-right side of the Safari window, then click the Adblock Plus extension's name to open it. Since the extension file is an Internet download, your Mac may ask you to confirm that you want to install Adblock Plus before actually installing it. You will most likely have to click Trust or Install from Developer when prompted to install this extension. Once this tab opens, Adblock Plus has been successfully installed. In order for AdBlock Plus to take effect, you'll need to close the Safari browser and open it again for AdBlock Plus to start blocking YouTube ads effectively. To close Safari:  Click Safari in the top-right. Click Quit Safari in the drop-down menu. Now that Adblock Plus is installed, YouTube videos should no longer display ads. While the ads on YouTube videos should no longer play, this might not completely block the ads that appear on youtube pages outside of the video.

Summary:
Open Safari. Open the Adblock Plus download page. Click Safari. Open the downloaded extension file. Follow any on-screen prompts. Close the Adblock Plus tab when it opens. Restart Safari. Watch YouTube videos without ads.