I did but I'm on if it can be done on a mobile bitcoin wallet app because I don't have a PC
You didn't mate. For smartphone were all included in that thread, perhaps you didn't read the complete first page.
DOWNLOAD MYCELIUM WALLET on play store.
Follow this guide on how to sign message with mycelium wallet.
https://bitcointalk.org/index.php?topic=990345.0#post_mycelium230 Remember to verify after sign otherwise Ognasty will reject it. Very simple.
Good luck.