If you don't have access to your P60 or P45, you can get your employment income and tax paid for given tax year from HMRC by following these steps:
Log in to https://www.gov.uk/personal-tax-account
Create an account if you've never been there before or alternatively log in
Beneath Income, select Pay As You Earn (PAYE)
Select Check previous tax years
It should now show your employer(s)... select Check the income details sent to us
At the bottom of the table, you’ll find a total row that should include Taxable income (£) and Income Tax paid (£)
Enter these details on the Tax tab of Earnr, under As an employee.
If you have multiple employers, you can repeat steps 5,6 and 7.