You are required to complete a Self Assessment tax return for every year (after you ceased to be a student) in which you received earnings from your Romanian employment.
Your employment details (the name of your employer, your pay before tax and the UK tax deducted) should be entered in the employment pages.
To claim any Foreign Tax Credit Relief, the relevant information should be entered in the foreign pages under the heading 'Foreign tax paid on employment, self-employment and other income'.
Regarding your years as a student, please see the following guidance:
DT16055 - Romania: Students and trainees