It is relatively straightforward to make the claim to be non-UK domiciled, and to claim the remittance basis on your tax return.
Your tax return needs to be completed and filed by 31 January following the end of each relevant tax year. This means that you should be able to assess whether your offshore income and gains for that tax year make claiming the remittance basis worthwhile.
Domicile and remittances are complex areas of law, and HMRC can and do make enquiries from time to time to ensure the claims are valid and the right amount of tax has been paid. HMRC may ask for more details around your family and personal circumstances, to satisfy themselves that you are indeed non-UK domiciled.
In many circumstances, your domicile position may be fairly self-evident. However, domicile is determined according to complex rules and if you believe that you have reason to claim non-dom status, you should always seek professional advice.