Hi,
The remittance basis is applied for by using SA109, where you declare the amount of unremitted income for the tax year, box 34, captial gains, box 35, and the type and breakdown of unremitted income / capital gains, box 40. You can see guidance here:
Resideence, remittance basis etc (2024)
If you are remitting, previously unremitted income, and, or capital gains in a later tax year, then you would declare this in SA106, foreign, or another relevant supplementary page. You can see guidance on the remittance basis here:
Residence, domicile and the remittance basis: RDR1
Thank you.