First, choose the ad blocker you'd like to install. There are a few available as of the time of writing this guide.
1Blocker Adamant Blockr Crystal Freedom Purify Silentium
For this guide, I'm going to use Crystal because it's free. Open the app store, and search for Crystal. Then click to install.
Enable Crystal or whichever content blocker you've downloaded. You can enable or disable each content blocker a simple toggle. After enabling it, you should immediately see it take effect.