It varies slightly depending on jurisdiction but broadly they follow the same process operationally.
You should report your variable income on the provided dashboard when prompted or whenever your income changes. You will select Income from the menu bar and "Variable Income" from the different income reporting options (often Annual Income, Variable Income, Not Employed). You will then be asked to enter your monthly income for a specific month, along with some other details and information. The ISA provider will then reach out monthly to inquire whether your income needs to be updated before the next payment is scheduled.
You may need to provide additional proof of income at a later date, for example your tax return.