Prerequisite: Please make sure have active CloudFlare account and you have added your WordPress Website to CloudFlare. Also, make sure you are using CloudFlare nameservers for your domain.
Step 01: Sign in to your CloudFlare account
Step 02: Make sure flexible SSL certificate is enabled under the “Crypto” tab for your selected website
Step 03: While you are already signed in to your CloudFlare account go to > My Profile > Scroll down to API Keys and locate Global API Key & copy the same key for further use inside your website.
Step 04: Now sign in to your respective WordPress Website Admin Page
Step 05: Now install the CloudFlare WordPress Plugin for your WordPress website. After installing plugin activate it for further use.
Step 06: Now go to CloudFlare WordPress Plugin settings page (You will find Plugin Settings Page under the ‘Settings’ tab in your WordPress Admin page inside the left sidebar).
Step 07: Now copy paste your CloudFlare email and Global API Key inside CloudFlare Plugin Home Page by clicking on ‘Sign in here’ link.
Step 08: Now inside plugin setting page go to settings tab at top > click to enable “Automatic HTTPS Rewrites” setting in the plugin
Step 09: Go back to your signed in Cloudflare account on cloudflare.com website. Now go to ‘Crypto’ tab at top > enable ‘Always Use HTTPS’ option
Step 10: All Done ‘Success’, You have enabled CloudFlare Flexible SSL Certificate for your website. To confirm refresh your website to see ‘lock sign’ near your preferred browser’s URL address.
Hope it helps!
Thanks & Regards