Once you've identified which pages have the worst CLS, you can use PageSpeed Insights to figure out what elements on the page you're analyzing are causing layout shifts. Enter the URL of the page into the box and hit Analyze. When it loads, it should provide a large report for the page. Look for Avoid large layout shifts listed under the Diagnostics area (halfway down the page).
After you determine what's causing the layout shift, your next step will be to figure out why these elements are shifting content and how to prevent it.
Oftentimes, to improve CLS on your site we recommend:
Ensuring that you specify the dimensions of images and other block items on your site. Including the dimensions for images, div tags, and other blocks of content allows the browser to plot the layout even if elements are slow to load into it.
Make sure there aren't any banners (like free shipping banners), alerts, or popups that move around on the visitor's screen. Even if these things don't shift content, we've found if they move, they will trigger layout shift.