When adding kilometer allowances, you must have a document from your driving diary in accordance with the instructions given by the tax authority.
The logbook must show:
start and end time,
start and end place,
purpose of the trip,
cars register number,
other possibly needed information (e.g. passengers on board)
Day allowance: e.g. travel ticker, calendar note, name of the person you were meeting.
Meal replacement: receipts of the food, proof of the need of compensation, e.g. calender note about a meeting.
Receipts: Keep the receipts of your purchases.
What documents do I need to have for expenses?