If you have already created an account on InfinityFree then the next steps are easy.
Go dash.cloudflare.com and create an account. Once you’ve done that, add your site. Cloudflare will give you two nameservers. Navigate to your domain provider and change the nameservers to the ones provided by Cloudflare. You will then have to wait for the nameservers to update, but once they do, Cloudflare will verify it and your site will be active on Cloudflare!
Hope this helps 