You can't declare it online via ROS from what I have seen, only pay it. I'm set up for CGT and the way I pay it and declare it is by just working out how much I owe, paying through the CGT portal on my account and then sending them a scan of the completed CG1 form and asking for my record to be updated to incorporate it. They always reply to me and acknowledge receipt.

When filling out my tax return for the year, I see nowhere to file it so I didn't include it - they have my CG1 forms and the tax has been paid so they have it anyway. I could be completely wrong in all of this, but I can't see anywhere to declare additional income from stock trading on tax return form 11.