If you’re employed: The lender will ask for written evidence e.g. payslips and/or your P60 for the past two years.
They’ll also probably write to your employer asking for confirmation.
Some lenders may accept income that’s not guaranteed eg commission, bonuses etc., though this would be exceptional.
If you’re self employed: Traditionally this was more difficult and as a result there are lenders who specialise in the self employed.
However, nowadays, many lenders will be interested in you.
You would need to show three years audited accounts. If you haven’t been in business long enough then the lender should accept a letter of confirmation from your accountant.