You do not need a perfect folder of paperwork. Start with enough information to make the first scenario useful.
A simple starting list
- Your usual take-home income.
- Regular household costs such as rent, mortgage, food, power and transport.
- Debt balances, interest rates and regular payments.
- Savings or investments you want included.
- A few annual or irregular costs, such as insurance, rates, repairs or gifts.
If a figure is missing, use a clearly labelled estimate. You can improve it later.
What to try next: Gather the three numbers that matter most to your current question. That is enough to begin.