To claim split year treatment, you will need to submit a self assessment tax return. You can do this either by downloading and printing a paper SA100, SA109 and any other relevant supplementary pages and posting the completed forms to HM Revenue and Customs BX9 1AS, or by purchasing 3rd party commercial software and submitting the tax return electronically.
Both options can be found at:
File your Self Assessment tax return online
You do not have to report overseas income received before you arrived in the UK. However any overseas income (including Trust income) received after you arrived in the UK should be declared. You should also declare in your tax return commission, cash-in-hand payments over the Trading Allowance of £1000, pension savings tax charges and SEISS grants and payments.
Self Assessment tax returns
Pension savings — tax charges (Self Assessment helpsheet HS345)
How to report your earnings from self-employment