You need to report your self-employment income by submitting a tax return after the end of each tax year. The tax year runs from 6 April to 5 April the following year.

The deadlines for submitting your return and paying any money you owe are:

paper tax returns: 31 October
online tax returns: 31 January
pay tax you owe: 31 January following the end of the tax year. There are rules surrounding making payments on account.

You can get more information on https://www.gov.uk/understand-self-assessment-bill/payments-on-account