Claiming previous years subscriptions

I am completing my self-assessment and want to claim for the trade union subscriptions paid for this year and last year (the union is on the approved list). Do I just add these together and put on the self-assessment form?

Hi,
You will need to amend your tax return from last year to claim the subscriptions payments incurred for that period. You can do this online in your persional tax account or by submitting an amendment in writing please see the guidance here: 
Self Assessment tax returns
Thank you.