To do this you would need to upgrade to the business plan ($200/month), then under Crypto click ‘Upload Custom SSL certificate’, and copy/paste the keys created by Lets Encrypt into Cloudflare. Then the Cloudflare server will be serving the Lets Encrypt certificate to visitors.
Note you would need to manually update the keys every year when the expire, as your Lets Encrypt control panel doesn’t automatically update the Cloudflare server.