Once logged into your Cloudflare account click (+ Add a Site) and type in your domain. With your site active click on DNS and adjust your records as shown below.
You will want to create a CNAME record with example.com (your domain) under Name and lb.wordpress.com under Content.
Create a second CNAME record with www under Name and example.com (your domain) under Content. This will forward http://www.example.com to example.com.
Allow a few minutes for Cloudflare to update the records.
Notice: Using Cloudflare in front of your WordPress.com site will affect WordPress.com stats. That information will now be in Cloudflare analytics under your active site. Information collected is different between WordPress.com stats and Cloudflare analytics.