1. Your group
The opening balance is the bank balance just before the first transaction in your file. Every month after that is worked out from the transactions.
2. Bank export
Export CSV from online banking (most banks: Activity, Download, CSV). Paste it here or pick the file. Needs a date, a description and an amount column, or debit and credit columns. Deposits positive, payments negative; tick the box if your bank does it the other way.
3. Category rules
One per line: text found in the description => Category. First match wins. Anything that matches nothing shows up as Uncategorized below, where you can assign it and a rule is written for you.
4. Budget (optional)
One per line: Category, amount for the year, income or expense. Categories without a budget line still show on the report; the tool guesses income or expense from the sign.