Create the proof
Log in to the Discourse instance's and add the proof to your About me.
After posting, copy the link to your profile page, it should end with your /u/USERNAME: this will be your PROFILE_URL.
Add the claim…
The claim looks as follows:
Make sure to replace PROFILE_URL.
…to a signature profile:
Follow the instructions provided here. The claim should look like:
…to an OpenPGP profile:
Follow the instructions provided here. The notation should look like: