Instructions for enabling DNSSEC with DNSimple and Cloudflare can also be found on DNSimple's blog here: http://blog.dnsimple.com/2015/11/ds-records-for-dnssec/
1. Click on the 'domain' tab in the upper-left navigation for the domain you are enabling DNSSEC
2. Scroll all the way to the bottom of the page until you find the 'DS record' option. Click 'Configure DS Record'.
3. Click on the 'Quick Entry' tab at the top.
4. In the Cloudflare dashboard, copy the 'DS Record' field
5. Paste the copied DS record into the DNSimple dashboard, and click 'Create DS Record'
That's it! You've setup DNSSEC.