It’s less complicated than it seems. The idea is, get your comment in the clipboard (by control-C copy), sign it with Kleopatra, and paste it back in for commenting, but bracket it with the <pre> and </pre> tags so WordPress doesn’t mutilate the headers.
1) Type in your comment, but don’t post it yet.
a. Copy your comment (select the text and hit Control C).
b. Run Kleopatra (or Alt-Tab over to it if it’s already up and running).
c. Click on the ‘Clipboard’ button on the top right of the Kleopatra dialog window.
d. Click on ‘OpenPGP-Sign’
e. If it’s not already the default, select your certificate.
f. Remember that passphrase you chose earlier? Time to enter that passphrase now!
g. If God is Good you will see ‘Signing Succeeded.’ The text in the clipboard is now signed! Click OK.
h. Alt Tab back to your browser and your unsigned comment. Wipe it out.
i. IMPORTANT. Type <pre> and press enter.
j. paste in your signed comment by pressing Control V.
k. IMPORTANT. Type </pre> and press enter.
2. Post your signed comment!