Capital Gains and Interest tax

I am new to UK, have a few questions about the tax. 1.) I saw that the Self Assessment has to be registered on October, however, i just decided to sold some of my overseas stock in March, can i still apply a Self Assessment to report this transaction? Will i get any penalties on this? 2.) I have a loss in stock too, how can i report this to HMRC? What documents do i need to provide?

Hi mingyeung1945,
Please refer to guidance here:
https://www.gov.uk/report-and-pay-your-capital-gains-tax.
If the losses are the same year, you will account for them when declaring the gain if for a previous year.
HMRC need to be made aware of the loss within 4 years of it arising.
You will need to send in a letter if this is the case giving details of date/price purchased and date/price sold and the name of the company the shares were in.
Thank you.