Configure the client. There will be interactive prompts for you to enter your email address, CloudFlare API key, and domains you want to update.
cloudflare-ddns --configure
After configuration, it would be in your favor to ensure that the update operation works as you expect, without errors.
cloudflare-ddns --update-now
Add a DNS update operation to crontab.
crontab -e
For example, if you want your DNS records to be updated at midnight every day, you would enter the following. This implies that, effectively, an external IP change would never impact connectivity to the server for more than 24 hours. You can make the update interval more or less aggressive as you see fit.
0 0 * * * /usr/local/bin/cloudflare-ddns --update-now > /dev/null 2>&1